The research lacks evidence, and therefore, its conclusions are doubtful.
| A．solid | B．fierce | C．severe | D．potential |
For many cities in the world, there is no room to spread out further, ________New York is an example.
| A．for which | B．in which | C．of which | D．from which |
Gun control is a subject _________Americans have argued for a long time.
| A．of which | B．with which | C．about which | D．into which |
She’ll never forget her stay there _________-she found her son who had gone missing two years before.
| A．that | B．which | C．where | D．when |
—What do you think of teaching, Bob?
—I find it fun and challenging. It is a job __________you are doing something serious but interesting.
| A．where | B．which | C．when | D．that |
Whenever I met her, ________was fairly often, she greeted me with a sweet smile.
| A．who | B．which | C．when | D．that |