Listen! I hear some birds ___________ the tree.A. sing in B. singing in B. sing on D. singing on
--- Sam, do you know when Alice ______to my party ? ---I think she will come when she ______free .
We have two rooms ________, but I can’t decide ___________.
---Doctor , I’m getting ________. --- Well , you’d better eat ______food.
The accident happened _____the old man _____ a cold morning .
This is Mr. Green’s ________visit to America. He has never been there before.