reference

xyu201

\"Tom had a similar problem when he arrived in India. After explaining
something in class, he asked his students if they understood.
They responded with many different nods and shakes of the head. He
assumed some people had not understood, so he explained
again. When he asked again if they understood, they did the same
thing.\"
Interchange 3, third edition, by: Jack C. Richards et al/ page21

What does \"the same thing\" refer to?
a) responded with many different nods and shakes of the head
b) respond with many different nods and shakes of the head
c) the responding with many different nods and shakes of the head
d) to respond with many different nods and shakes of the head
or maybe another option(?)