表面热处理就是改变钢材表面的化学成分，从面改变钢材表面的性能。
参考答案：对
--- Are ______ Kate’s ______?
--- Yes, they are.
A．this, book
B．that, book
C．these, book
D．those, books
If you have any comment about these clauses, do not hesitate ( ).
A．make
B．making
C．being made
D．to make
