. The matter had better _______ as it is.
| A．leave | B．being left | C．leaving | D．be left |
Thanks to Dr Yuan’s research, the UN has more tools in the battle to rid the world _____ hunger.
| A．of | B．on | C．off | D．at |
______ is even more important is that the earth cooled down, water began to appear on its surface.
| A．When | B．That | C．Which | D．What |
Jane _____ these families of chimps for many years and helped people understand how much they behave like humans.
| A．had studied | B．studied | C．studies | D．has studied |
It is so wet there _____ the trees are extremely tall, some measuring over 90 meters.
| A．what | B．which | C．that | D．why |
Many _______ of years later, the first extremely small plants began to appear on the surface of the water.
| A．millions | B．million | C．million of | D．millions of |