I can still remember that it is since the 1990’s, _____ we began to use cell phones,_______ the number of mobile users has greatly increased.
There was an excited expression ______ his face when he heard the good news.
Milk will turn sour soon on hot days ______ it is stored in the refrigerator.
You'd better not ride the bicycle on snowy days; you ______ fall.
—Hi, Jack.You look a bit fat now. —Yes, I ______ five pounds recently.
Every book ______ from the library should be returned on time.