Becareful,_______you'llfalldown.
A、onthecontraryAsyoulikepopmusic,__________Ilikeliterature.
Theproblem________whenthestudentsrefusedtodotheirhomework.
Constructionoftherailwayshelpedtodevelopthenational_______ofTanzaniaandZambia.
Thefarmerhadtowearheavybootsinthewinterbecausethefieldsweretoowetand________.
Wemust______thattheexperimentiscontrolledasrigidlyaspossible.
