—Will you stay for some more days? —Sorry, I_____. My mother called to ask me to go back at once. 

—Why is online shopping getting more popular? —I think it _________ save a lot of time.

Students ought ______ the truth.

You had better ________some food and drinks before the field trip ________.

 --- Another cup of coffee? ---No. thanks. I _____ be off. Marry is waiting for me.