词形转换（本题有10小题，每小题1分，共10分）
When it is very, very cold, there’s     (ice) on lakes. It might be safe to          _______(skate) on lakes, but be very careful!     (although) it may feel cold, it might not be safe. The        (temperature) has to be at least       (minus) one or two _______(degree) or even lower for several   (week) and the ice will be thick enough. Falling through the ice is    （dangerous）. I am not      .(joke) You had better_____(wear) kneecaps(护膝) .