About how many elements _____ make up most of the substances we meet in everyday life
If you had told him the truth, he ______ so angry now.
Teaching is a difficult job ______ requires great skill and patience.
Drivers must stop when the traffic lights _____ red.
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.