【单选题】

The first two stages in the development of civilized man were probably the invention of primitive weapons and the discovery of fire, although nobody knows exactly when people acquired the use of (1) .

The (2) of language is also obscure. No doubt it began very gradually.Animals have a few cries that serve (3) signals, (4) even the highest apes have not been found able to pronounce words (5) with the most intensive professional instruction. The superior brain of man is apparently (6) for the mastering of speech. When man became sufficiently intelligent, we must suppose that he (7) the number of cries for different purposes. It was a great clay (8) he discovered that speed could be used for narrative. There are those who think that (9) picture language preceded oral language.A、man (10) a picture on the wall of his cave to show (11) direction he had gone, or (12) prey he hoped to catch. Probably picture language and oral language developed side by side. I am inclined to think that language (13) the most important single factor in the development of man.
Two important stages came not (14) before the dawn of written history. The first was the domestication of animals; the second was agriculture.Agriculture was (15) in human progress to which subsequently there was nothing comparable (16) our own machine age.Agriculture made possible (17) immense increase in the number of the human species in the regions where it could be successfully practiseD、 (18) were, at first, only those in which nature fertilized the soil (19) each harvest.Agriculture met with violent resistance from the pastoral nomads, but the agricultural way of life prevailed in the end (20) the physical comforts it provideD、
A．should gradually increase
B．gradually increase
C．gradually increased
D．has gradually increased