Which of the following words can NOT be added the prefix "un_______" to give it the opposite meaning？（ ）
A．happyB．healthyC．usualD．polite
Would you please ____________ the light？ I can't sleep well with it on．（ ）
A．turn onB．turn off
C．turn toD．turn around
Love your parents __________ they are alive． Don't wait until it is too late．（ ）
A．whileB．thoughC．becauseD．unless
The flying squirrel might be one of __________ animals you could meet during the trip．（ ）
A．strangeB．stranger
C．strangestD．the strangest
Some people don't show their talents at the very beginning．
I agree． Even Einstein ____________ read until he was seven．（ ）
A．can'tB．mustn'tC．couldn'tD．needn't
Keep quiet! I need complete__________ when I'm working．
Sorry， dad． I won't make any noise again．（ ）
A．trustB．silenceC．controlD．strength