[单项选择]Some of these studies (have shown) that although some people have trouble (to fall asleep), others have an (equally) difficult time (waking up). A. have shown B. to fall asleep C. equally D. waking up
参考答案:B 答案解析:改为falling asleep,have trouble/difficulty (in) doing sth.类似结构还有It’s no use (useless/pointless) doing