问题
选择题
Mr. Green _______ his lessons when I went into his office.
A．prepares
B．prepared
C．is preparing
D．was preparing
答案
答案：D
题目分析：句意：格林老师在我走进他办公室时他正在备课。when从句中用的是一般过去时态，所以根据句意主句应用过去进行时态，故选D。
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