Most birds find it safe to sleep in the trees, but __________ they have eggs or young chicks, they don’t use a nest.
| A．why | B．how | C．unless | D．where |
23. If you want to do the experiment again, you had better be more careful________ you made a mistake.
| A．when | B．why | C．where | D．That |
22. All of us want very much to see these recommended movies, especially _____ you referred to just now.
| A．as | B．which | C．the one | D．One |
21. Perseverance is a kind of quality, and that’s ______ it takes to do anything well.
| A．what | B．that | C．which | D．why |
20. —Why was __ you lost all your keys？
—It was because of my carelessness.
| A．that | B．what | C．it that | D．One |
19. To work at home has been made _____ with the development of the Internet.
| A．it probable | B．it likely | C．likely | D．possible |