A ruler is 2 dollars and a pen is 5 dollars. How much are they? -They are

_________ did you do with old books?— I sent them to children in poor areas.

________ have you become so good at English this term ?—By studying grammar and reading English novels.

______ classes do you have in the morning?—Four classes.[ ]

Listen！Helen is singing in the next room.—It _____be Helen .She has gone to Beijing.