单项选择题
Earthworms occur ______ adequate moisture and food and the necessary soil conditions are found.
单项选择题 The tree, the branches ______ are almost bare, is a very old one.
单项选择题 Without computers，we ______ the tremendous medical advancement in the last few decades.
单项选择题 French is not his mother ______ but he can speak it excellently.
