【2015·安徽】______ scientists have learned a lot about the universe, there is much we still don't know.
| A．Once | B．Since | C．Though | D．Unless |
As is known to us all, _____ tiger is in _____ danger of becoming extinct.
| A．the, a | B．the, 不填 |
| C．a, 不填 | D．不填, the |
This is _____ best kind of _____ pen you can get here.
| A．the, the | B．the, a |
| C．the, 不填 | D．a, the |
How strange! These years my birthday always falls _____.
| A．on the Sunday | B．on a Sunday |
| C．on Sunday | D．at a Sunday |
As _____ writer, he was _____ complete failure.
| A．a, a | B．a, the |
| C．不填，不填 | D．a, 不填 |
The operation is _____ success and the patient is now out of _____ danger.
| A．a, the | B．a, 不填 |
| C．不填, the | D．不填，不填 |