阅读下面短文，从每题所给的A、B、C、D四个选项中选出最佳选项。

It's an English class. Mr Smith teaches us very well. He asks Li Lei from China," What's six and four?"

"Of course it's ten," Li Lei answers. "And ten and three is thirteen."

Then Susan from the USA answers," I know ten and three is thirteen."

"OK. You are all very good. Now let's do it freely. You can ask your partner to answer two numbers," says Mr Smith.

When they are having a good time, their maths teacher, Mr Black comes in. He is very surprised," What class is it? Isn't it Monday today? I don't have a lesson today. "

"Ha! Ha!" All the students laugh(笑).

"I'm sorry, Mr Black. We are having an English number lesson," says Mr Smith. Then Mr King, Ms Green and Ms Brown come, too. They all laugh, too.