11)
a. at the Zoo
b. in an exam
c. in the park
d. before you eat them
e. at night
f. in cold weather
12)
can't/must/can/can/mustn't/mustn't/can/can't/must
13)
a. I have a sore throat. I should not...
b. Children mustn't...
d. Your room is untidy, you should...
c. You mustn't...
e. This is a secret. You mustn't...
f. You must not...
g. She hasn't got a lot of time, she should...