The missing boys were last seen ________ near the river.
A
所填的词为主语补足语，因其后有明确的地点，补足语表示动作当时正在进行中，所以只能使用playing。
Poor speaker of English though he was at the time, he still managed to make himself ________.
The policeman came up to the lonely house with the door _______, _______ there for a while and then entered it.
Thepolicemancameuptothelonelyhousewiththedoor_______,_______thereforawhileandthenenteredit.
______ a little earlier, I would have caught the train.