一、用括号内所给词的正确形式填空。
1. I'll go to the zoo if it ____ (be) sunny tomorrow.
2. Frank ____ (go) to see his grandma if he ____ (be) free tomorrow.
3. If you ____ (work) harder, you will catch up with us soon.
4. I will keep waiting for my friend unless he ____ (ask) me to leave.
5. The students ____ (not plant) trees if it ____ (rain) tomorrow.

1. is 2. will go; is 3. work 4. asks 5. won't plant; rains