Question

what are you doing? i'm reading a book by Mark Twain. A.written B. writing C.to write D.write

look! the girls are busy English over there
A.practising speaking B.practising speak

Answer 1

1. 选A
I'm reading a book written by Mark Twain.
这里book 和write是被动关系，所以要用过去分词
2. 选A
busy doing sth 意思是：忙着做某事，相当于busy in doing sth.
practise doing sth.意思是：练习做某事

Answer 2

A 应用被动式“written”。即：我在看一本Mark Twain写的书
A固定搭配practice doing sth

Answer 3

A.
A.