四、it类. Everyone knows that _______ is dangerous to play with fire, but _______ is difficult is to prevent children from playing with fire.
Some of the students are not sure ___ spelling the word, while others are not sure ___ the use of some grammatical rules.
He advised me to read an artile ___ reading ___ an American writer ___ senior Book 3.
Who is ___ the school?
The PLA man made a dive ___ the smoking box.
The naughty boy could hardly tell me the reason ___ being absent ___ school.