The correct answer is: B. Are nutrient-poor environments.
The open oceans are nutrient-poor environments because they are far from land, where most of the nutrients are found. The water in the open oceans is also very deep, which makes it difficult for nutrients to reach the surface. This lack of nutrients limits the growth of plants and animals in the open oceans.
Option A is incorrect because the open oceans are not rich in silica and iron. Silica is a type of sand that is found in soil and rocks. Iron is a mineral that is found in rocks and soil. Both of these substances are found in much higher concentrations on land than in the open oceans.
Option C is incorrect because the open oceans do not contain unlimited nitrogen. Nitrogen is a gas that is found in the atmosphere. It is also found in soil and rocks. However, the amount of nitrogen in the open oceans is much lower than the amount of nitrogen on land.
Option D is incorrect because the open oceans are not nutrient-rich environments. The lack of nutrients in the open oceans limits the growth of plants and animals.