they stoped ________,butthere was no sound?listening hearing

Seeing their teacher____into the classroom, they stoped ____ at once.
A.walk, telling
B.entering,to speak
C.enter, to tell
D.walking,talking
see sb doing sth 与see sb to do sth有何区别
stop doing sth 与stop to do sth 有又何区别
我觉得有点难耶 ～ 不会不会 有点 理解不了非谓语