The missing boys were last seen ________ near the river.
A、playingA
所填的詞為主語補足語，因其后有明確的地點，補足語表示動作當時正在進行中，所以只能使用playing。
________hehasmanyfriends,heisoftenfeelinglonely.
Thehousewasquietinthenight,________theoccasionalcoughingfrommythroat.
Thegovernmentcalledoutpolicementosuppresstheriot.
Scientistsdoubtwhetherit’s________toregulateone’sbiologicalclockbydrinking.
______isrecognizedasthenationalflag-carrier.
