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 。
Scientistsdoubtwhetherit’s________toregulateone’sbiologicalclockbydrinking.
In Europe _______ are the two leading contenders for a global reservations system.
PoorspeakerofEnglishthoughhewasatthetime,hestillmanagedtomakehimself________.
______thenewplancanbecarriedoutwillbediscussedatthemeetingtomorrow.
The Netherlands is the only country in Europe which permits euthanasia, ________ it is not technically legal there.
