【正确答案】 A

【答案解析】[听力原文] 18-20
Scientists believe that when the oceans were young，they contained only a trace of salt，and that the level of salinity has been going up gradually(18)(19)．Over the years，rains have been wearing away the rock mantle of the continent．Minerals released by this erosion have been carried to the sea by the flow of river water．This water adds several billions tons of salt to the ocean annually．There are other sources of oceanic salinity too；one source is underwater volcanic action(20)．These eruptions occurred along the mid-ocean ridge and release great amount of salt into the sea each year．However，these sources don't increase the actual mineral composition of the water by much because the enormous chemical requirements of marine life．Oysters，clams，snails and other mollusk use salt to build shells and skeletons．Coral reds also consume large amount of salt．Also，some salt leaves the ocean through the process of evaporation．As salts escape into the atmosphere，they go into the creation rain drops，this is why the coast receives more rain than land-lock aeras．You see salt exists in the atmosphere near the ocean，thereby attracting the increased moisture and producing rain．
文章开篇就点明了主题，即科学家认为海洋形成初期盐分并不多，海洋的盐分是随着时间的推移逐步增加的，整篇文章都在讨论海洋的盐分，因此答案是A。
[名师点津]
1．a trace of：意为“一丝，些许”，表示程度。
2．oceanic salinity：海洋的盐度。