Rewrite the sentences according to the instructions in the brackets (按要求改写下列句子，每空一词）
【小题1】You mustn't leave rubbish on the ground. （改为同义句）
__________ ___________ rubbish on the ground.
【小题2】We mustn't bring balls into the class building. （就画线部分提问）
__________ __________ we do?
【小题3】There isn't anything on his desk. （改为同义句）
There __________ __________ on his desk.
【小题4】We usually walk to school.（改为同义句）
We usually go to school ___________ ____________.
【小题5】My best friend is 12 years old. （就画线部分提问）
___________ ____________ is your best friend?
【小题6】He often helps me. I often help him. （改为同义句）
We often help _____________ ____________.