The maths problem remained confusing to the boy although __________ over and over.
| A．explaining | B．being explained | C．explained | D．having explained |
Over the past decades, sea ice______in the Arctic as a result of global warming.
| A．had decreased | B．decreased |
| C．has been decreasing | D．is decreasing |
Bill was doing a lot of physical exercise to build up his______.
| A．energy | B．strength | C．force | D．power |
—I wonder ______ you’ll water this kind of flower.
—Every other day.
| A．how often | B．how long | C．how soon | D．how much |
. She brought with her three friends, none of ______ I had ever met before.
| A．them | B．who | C．whom | D．these |
. — Why does the Lake smell terrible?
—Because large quantities of water______.
| A．has polluted | B．have been polluted |
| C．has been polluted | D．is being polluted |