Themissingboyswerelastseen________neartheriver.
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 。
Scientistsdoubtwhetherit’s________toregulateone’sbiologicalclockbydrinking.
Motivationandpersonalitymay________thevaryingdegreesofsuccessinone’scareer.
Thepolicemancameuptothelonelyhousewiththedoor_______,_______thereforawhileandthenenteredit.
______thenewplancanbecarriedoutwillbediscussedatthemeetingtomorrow.