All the plants and animals which we know of on earth have to breathe,and so can live only on planets which have suitable atmospheres,Mars and Venus both have atmospheres,but neither is of the sort which would support earth-type men or animals.

• 我们知道地球上的一切动、植物都必须呼吸，因而只能生活在有合适的大气层的行星上。虽然火星和金星有大气层，但是都不是那种能维持地球上的人或动物生存的大气层。