【单选题】

The passage suggests that if there were a slight global warming at the present time, it would be ______.
A．easy to measure the exact increase in temperature because of the abundances of temperature recording stations throughout the world

B．difficult to prove that the warming was caused by the burning of fossil fuels
C．easy to demonstrate the effects of the warming on the water vapor in the atmosphere
D．difficult to measure the increase of CO₂ in the atmosphere because of local variations in amount