问题
选择题
Scientists say it will be five or six years _____ it is possible to test this medicine on human patients.
A．before
B．that
C．since
D．when
答案
答案：A
此题考查时间状语从句的引导词。由It + be + 时间段 +before 从句“要过多久才…”可知，此处用before 。
