She often keep the office clean when she was at work.中的動詞要怎樣

題目：

She often keep the office clean when she was at work.中的動詞要怎樣變化才對
原題是he often -the office -when she was at work.
Akeeps；cleaned Bkept;cleaned Ckept;clean Dkeep;clean

解答：

這樣的話選C
因爲後面的從句「when she was at work"是過去式,主句和從句時態是一致的,那麼主句也必然是過去式,排除了A D
然後Keep st.adj也是固定搭配,clean本身就是形容詞.不能再加ED.那麼最後選C