Scientists say it may be five or six years it is possible to test this medicine
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Scientists say it may be five or six years ____ it is possible to test this medicine on human patients.
A．since B．after C．before D．when
试题答案
C
解析:
此题考查连词以及对句式的理解。It is + 一段时间+before，意为“要（多长时间）……才能……”。