Carbon dioxide, which makes a ______ between us and the sun, prevents heat from getting out of the atmosphere easily, so the earth is becoming warmer.

Shelly had prepared carefully for her biology examination so that she could be sure of passing it at her first         .

At the meeting, the old man ________ some good advice about bringing down the market price of housing.

At the meeting they discussed three different ________to the study of math.

We still have a long way to go to solve all the problems _____________the Yangtze River.