I just don’t understand _______ that prevents so many Americans from being as happy as one might expect.
| A．why it does | B．what it does |
| C．what it is | D．why it is |
Life is like _________ ocean; only ________ strong-willed can reach the other shore.
| A．an; a | B．an; the |
| C．the; the | D．the; a |
The two girls are so alike that strangers find________difficult to tell one from the other.
| A．it | B．them | C．her | D．that |
The manager had fallen asleep where he_______，without undressing.
| A．was laying | B．was lying |
| C．had laid | D．had lied |
Give yourself 10% of________is left for your work and let me have the rest.
| A．which | B．that | C．what | D．where |
The school has developed an online system________ students can choose their own teachers.
| A．which | B．when | C．that | D．where |