单项选择题
A．When it is breeding. B．When it feels threatened by humans in its territory. C．When its offspring is threatened. D．When it is suffering from illness.
A．Its chemical elements must be studied. B．Its temperature must be lowered. C．Big spaceships must be built. D．Its atmosphere must be changed.
A．Study the case carefully beforehand. B．Stick to a set target. C．Appear friendly to the other party. D．Try to be flexible about their terms.