20. By the 1880’s, worlddemand for cotton had begun fall off.
答案是D,但是我觉得B单数可数名词不能单独存在,另外C在句中做什么成份呀,我觉应该是demanded for
22. At present, about 300 different varieties of dinosaur have been identified from bones found on every of the continents.
答案是d,但是我觉得b也错了
34. One of the most powerful tools available to labor unions has been the ability to strike in order to settleits disputes with management.
答案是d,怎么改?
thanks a lot