Your school things should _____.
A.be looked after well B.be looked well after
C.be taken good care D.be taken care of well
No one has yet succeeded in explaining the _____ of how life began.
A.puzzle B.problem C.cause D.reason
The missing boys were last seen ______ near the river.
A.playing B.to be playing C.play D.played
说明理由,