It is usual for young children to ask a lot of questions.       APT

=> Young children____________________________

7/8 x 4/21 + 2/3

I went to school in this building.       USED

=> This is the building__________________ to school.

I haven't always smoked much.     USE

=> I_____________________so much.

His father would disappear for days.     HABIT

=> His father had______________for days