Themissingboyswerelastseen________neartheriver.
A、playingA
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 。
China is a mountainous country.Hills,mountains,and plateaus cover _______ of the total area.
Scientistsdoubtwhetherit’s________toregulateone’sbiologicalclockbydrinking.
______ is recognized as the national flag-carrier.
He found _______ difficult to study maths.
Motivationandpersonalitymay________thevaryingdegreesofsuccessinone’scareer.
