I ____________ for about an hour and a half when suddenly I slipped and fell down a riverbank.

I ____________ for about an hour and a half when suddenly I slipped and fell down a riverbank. A: had been walking B: walked C: had walked D: was walking

以下句子中，结构为“主语—动词（单宾语及物）—宾语”的是： A: He worked out the difficult problem. B: They walked quickly along the riverbank. C: The letter lay open on his desk D: The storm will continue.