Use the reading below to answer the question. The layer of soil above the permafrost is too shallow for plants with deep roots to live. Grasses and shrubs can survive there because they have shallow roots. A sheet of mosses and lichens grows beneath these plants. When the soil above the permafrost thaws, the soil becomes muddy. Muddy soil is an excellent place for insects, such as mosquitoes, to lay eggs. Many birds spend the summer in the tundra to feed on these insects.

Based on the passage, what is one reason for the lack of trees in the tundra?

A.
soil depth

B.
amount of rainfall

C.
temperature range

D.
amount of sunlight