根据下面资料，回答
That year, in the local school, there was a new math teacher, as well as some new pupils. One of the new kids was the stupidest child that anyone had ever seen. It made no difference how quickly or how slowly they tried explaining numbers to him; he would always end up saying something enormously stupid. Like two plus two was five, seven times three was twenty-seven, or a triangle had thirty comers, etc.
Before this boy arrived, math lessons had been the most boring of all. Now they were great fun. Encouraged by the new teacher, the children would listen to the pieces of nonsense spouted by the new kid, and they would have to correct his mistakes.
Whenever the new teacher asked questions, the stupid kid would stand up but made the wrong answers, the other students all wanted to be the first to find his mistakes, and then think up the most original ways to explain them. To do this they used all kinds of stuff: sweets, playing cards, oranges, paper planes, etc. It didn′t seem like any of this bothered the new kid.
However, little Lewis was sure that it was bound to make him feel sad inside. Lewis was sure he would see him crying. So, one day, he decided to follow the new kid home after school. On leaving school, the new kid walked a few minutes to a local park, and there he waited for a while, until someone came along to meet him.
It was the new teacher!
The teacher gave the new kid a hug, and off they went, hand in hand. Following from a distance, Lewis could hear they were talking about math.
What does the underlined word "this" in the third paragraph refer to?
A.To find the new kid' s mistakes
B.To think up the most original ways to explain
C.To use all kinds of stuff
D.To follow him home after school