The professor said he wouldn’t attend the opening ceremony ____.

Storms most commonly occur_______a warm airmass and a cold air mass touch each other

What about _____ English-Chinese dictionary on thebookshelf—In my opinion, it’s _____ useful tool book for myEnglish learning.

The problem referred to at the class meeting, if ______________ in time, will make a bad effect on everyone in our class.

You can take away _________ you like most.