Some of the students are not sure ___ spelling the word, while others are not sure ___ the use of some grammatical rules

Which do you prefer, soda or coffee—I like ______ of them. Tea is my favorite.

Why was he unhappy yesterday? —A letter from home ______ an attack of homesickness.

That’s the reason ___________ he is leaving so early[ ]

Are you getting on well with Tom—Not very well, but we used to be friends.The more I know him, ________ I like him.