单项选择题
A．to B．until C．up D．onto
A．Taking out heavy things. B．Moving in light-weight furniture. C．Equipping the house with stones. D．Covering windows.
A．Tightly sealed house may be harmful in another aspect B．The house is unsealed to prevent the air from leaking C．The air always leaks from cracks D．New house is worse than old one in the aspect of indoor pollution