【单选题】

Anthropologists, psychologists and others have begun seeking the roots of ambition in family, culture, gender, genes and more. They have by no means thrown the curtain all the way back, but they have begun to part it.

If humans are an ambitious species, it’s clear we’re not the only one. Many animals are known to signal their ambitious tendencies almost from birth.Even before wolf pups are weaned, they begin sorting themselves out into alphas and all the others. The alphas are quicker, more curious, greedier for space, milk, Mom--and they stay that way for life.Alpha wolves wander widely, breed annually and may live to a geriatric 10 or 11 years olD、Lower-ranking wolves enjoy none of these benefits--staying close to home, breeding rarely and usually dying before they’re four.
Humans often report the same kind of temperamental determinism. Families are full of stories of the inexhaustible infant who grew up to be an entrepreneur, the phlegmatic child who never really showed much go.But if it’s genes that run the show, what explains identical twins--precise genetic templates of each other who ought to be temperamentally identical but often exhibit profound differences in the octane of their ambition
Ongoing studies of identical twins have measured achievement motivation--lab language for ambition--in identical siblings separated at birth, and found that each twin’s profile overlaps 30% to 50% of the other’s. In genetic terms, that’s an awful lot--"a benchmark for heritability", says geneticistDean Hamer of the U.S. NationalCancer Institute.But that still leaves a great deal that can be determined by experiences in infancy, subsequent upbringing and countless other imponderables.
Some of those variables may be found by studying the function of the brain.At Washington University in St. Louis, Missouri, researchers have been conducting brain imaging to investigate a trait they call persistence--the ability to stay focused on a task until it’s completed just so--which they consider one of the critical engines driving ambition.
The researchers recruited a sample group of students and gave each a questionnaire designed to measure persistence level. Then they presented the students with a task--identifying sets of pictures as either pleasant or unpleasant and taken either indoors or outdoors--while conducting magnetic resonance imaging of their brains. The nature of the task was unimportant, but how strongly the subjects felt about performing it well--and where in the brain that feeling was processed--could say a lot. In general, the researchers found that students who scored highest in persistence had the greatest activity in the limbic region, the area of the brain related to emotions and habits. "The correlation was .8 [or 80%]," says professor of psychiatry RobertCloninger, one of the investigators. "That’s as good as you can get."
It’s impossible to say whether innate differences in the brain were driving the ambitious behavior or whether learned behavior was causing the limbic to light up.But a number of researchers believe it’s possible for the nonambitious to jump-start their drive, provided the right jolt comes along. "Energy level may be genetic," says psychologist Simonton, "but a lot of times it’s just finding the right thing to be ambitious about." Simonton and others often cite the case of FranklinD、Roosevelt, who might not have been the same President he became--or even become president at all--had his disabling polio not taught him valuable lessons about patience and tenacity.
The link between ambition and persistence is like the one between ______ .
A、a car and its engine
B．one car and another car running after it
C．two cars of the same brand
D．two engines combined