铁是地壳的元素之一。

Iron is one of the ___elements of the Earth's crust.

汽车产生二氧化碳，这种化学元素的结合污染我们的环境并促进温室效应。

Cars produce CO2 and this combining of chemical ___elements pollutes our environment and promotes the Greenhouse effect.

氢是最轻的元素。

Hydrogen is the lightest ___element.