单项选择题
Racing the clock every day is such an exhausting effort that when I actually have a few free moments, I tend to collapse.（）
A. fall down B. fall ill C. fall out D. fall behind
单项选择题 It is deemed as impolite to leave a party without saying "good-bye" to the host.（）
单项选择题 （）to some parts of South America is still difficult, because parts of the continent are still covered with thick forests.
单项选择题 The flowers in the vase （）because they had no water.
