【推荐1】Tom ______ into the house when no one ______.

A．slipped; was looking	B．slipped; had looked
C．had slipped; looked	D．was slipping; looked

【推荐2】—Have you told Dr. Greenall our latest achievement in the vaccine research?
—Not yet. When I ________, he ________ a speech, so we had no time for a few words.

A．arrives; will give	B．arrived; was giving
C．arrives; is giving	D．arrived; has given

【推荐3】—Why are you so late today?
—Three buses went by without stopping while I ________ at the bus stop.

A．am waiting

B．wait

C．have waited

D．was waiting