问题

Scientists say it may be fiveyears ______ it is possible to test this medicine on human patients．

A:since
B:until
C:before
D:when

正确答案是：

同类题型

Li Lei is seen ___his homework every day.

Look! A pair of sport shoes ________under the bed.

下列各句中，表达得体的一句是（）

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He expressed a hope _______ he would visit China again the next year, and this was the very hope _______ he would express as soon as he arrived in the country.