Everyone knows that _________ is dangerous to play with fire, but _________ is difficult is to prevent children from playing with fire.

A. it, it    B. what, what C. it, what      D. what, it

 Everyone knows that _______ is dangerous to play with fire, but _______ is difficult is to prevent children from playing with fire.

A. it, it                    B. what, what

C. it, what               D. what, it

Everyone knows that _______ is dangerous to play with fire, but _______ is difficult is to prevent children from playing with fire.

Everyone knows that________ is dangerous to play with fire, but________ is difficult is to prevent children from playing with fire.

Everyone knows that________ is dangerous to play with fire, but________ is difficult is to prevent children from playing with fire.