A. sing B. is singing C. to sing D. sings
A. slept B. sleeping C. are sleeping D. sleep
A. to enter B. entered C. enter D. enters
A. to stay, do B. stay, do C. to stay, to do D. stay, to do
A. has changed B. is changed C. is changing D. changed
A. help B. helping C. helps D. to help