ign language like English is difficult to learn, soyou ____________ work too hard

What does the sign over there read?—“No person __________ smoke or carry a lighted cigarette，cigar or pipe in this area.”

There is only one of the oldest customs that ________ in this district up to now.

Jack _______ goes shopping, because he is ______ busy working.

The book is useful because it gives us advice on how to improve memory.