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 |
As the immune system weakens, it loses the ability ________ illnesses.
| A．fighting | B．fought |
| C．to fight | D．to be fought |
My mother went to the private school in person ________ that I would be safe there.
| A．confirming | B．having confirmed |
| C．to confirm | D．confirmed |
China is expected to export 1 billion mobile phones this year, ________ by a surge in demand for smartphones worldwide.
| A．drive | B．driving | C．to drive | D．driven |
Misunderstandings ________ from lack of social communication, unless ________ properly, may lead to serious problems.
| A．arisen; handling | B．arising; handled |
| C．rising; handled | D．risen; handling |
The text is too difficult. The explanation of our teacher will help make it easy ________.
| A．to understand | B．to be understood |
| C．understood | D．understanding |