6. A study was designed to establish what effect, if any, the long-term operation of offshore oil rigs had on animal life on the bottom of the sea. The study compared the sea-bottom communities near rigs with those located in control sites several miles from any rig and found no significant differences. The researchers concluded that oil rigs had no adverse effect on sea-bottom animals.
Which one of the following, if true, most seriously weakens the researcher’ conclusion?
(A) Commercially important fish depend on sea-bottom animals for much of their food, so a drop in catches of these fish would be evidence of damage to sea-bottom communities.
(B) The discharge of oil from offshore oil rigs typically occurs at the surface of the water, and currents often carry the oil considerable distances before it settles on the ocean floor.
(C) Contamination of the ocean floor from sewage and industrial effluent does not result in the destruction of all sea-bottom animals but instead reduces species diversity as well as density of animal life.
(D) Only part of any oil discharged into the ocean reaches the ocean floor: some oil evaporates, and some remains in the water as suspended drops.（B）
(E) Where the ocean floor consists of soft sediment, contaminating oil persists much longer than where the ocean floor is rocky.
觉得奇怪，怎么会是b呢，b不是加强吗？只在表面证明深海不会受污染啊。
我选c，虽然鱼没差别但种类和密度都少了，影响了深海鱼
nn赐教