Read the following passage and mark the letter A, B, C, or D on your answer sheet to indicate the correct answer to each of the questions from 36 to 42.

According to the controversial sunspot theory, great storms on the surface of the sun hurl streams of solar particles into the atmosphere, causing a shift in the weather on earth.

A typical sunspot consists of a dark central umbra surrounded by a lighter penumbra of light and dark threads extending out from the center like the spokes of a wheel. Actually, the sunspots are cooler than the rest of the photosphere, which may account for their color. Typically, the temperature in a sunspot umbra is about 4000 K, whereas the temperature in a penumbra registers 5500 K, and the granules outside the spot are 6000 K.

Sunspots range in size from tiny granules to complex structures with areas stretching for billions of square miles. About 5% of the spots are large enough so that they can be seen without instruments; consequently, observations of sunspots have been recorded for several thousand years.

Sunspots have been observed in arrangements of one to more than one hundred spots, but they tend to occur in pairs. There is also a marked tendency for the two spots of a pair to have opposite magnetic polarities. Furthermore, the strength of the magnetic field associated with any given sunspot is closely related to the spot's size.

Although there is no theory that completely explains the nature and function of sunspots, several models attempt to relate the phenomenon to magnetic fields along the lines of longitude from the north and south poles of the sun.

What is the author's main purpose in the passage?