---What does PM2.5 mean?
--- It’s a professional ______ indicating the condition of air pollution.
| A．number | B．mark | C．term | D．item |
If you don't go， neither ____.
| A．shall I | B．do I | C．I do | D．I shall |
Only by practising a few hours every day ____ be able to master the language.
| A．you can | B．can you | C．you will | D．will you |
Not until I began to work ____ how much time I had wasted.
| A．didn't I realize | B．did I realize | C．I didn't realize | D．I realized |
I want to be left alone ___ things out.
| A．think | B．thought | C．thinking | D．to think |
Before __ the trunk must be checked.
| A．used | B．to use | C．being used | D．to be used |