Before writing a book, the first thing is considering what to say.

A．you must first ponder what to say and what not to be said carefully.

B．it's extremely necessary that you know what to say.

C．the first thing is to consider what to say.

D．you must first ponder what to say and what not to say.

粉磨机的种类有：球磨机、（）、平盘式磨机和风扇式磨机等。