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。
______isrecognizedasthenationalflag-carrier.
Heshookhandswiththevisitors________astoconveyhisapology.
Chinaisamountainouscountry.Hills,mountains,andplateauscover_______ofthetotalarea.
Bythemiddleofthe21stcentury,thevastmajorityoftheworld’spopulationwillbelivingincities_______inthecountry.