Sunspots It’s not surprising that sunspots ( 太阳黑子）were observed by ancient astronomers (天文学家）.The largest sunspots on the sun can be seen without a telescope. It was not until the inventiion of the telescope ( 望远镜）in the early 17th century， however, that systematic studies of sunspots could be undertaken. The great astronomer Galileo was among the first to make telescopic observations of sunspots. Sunspots are regions of extremely strong magnetic fields ( 磁场） found on the sun’s surface. A sunspot has a dark central core known as the umbra. The umbra is surrounded by a dark; ring called the penumbra, where the magnetic field spreads outward. Sunspots appear dark because they are giving off less radiation. They are cooler than the rest of the sun’s surface. Sunsipots are frequently observed in pairs or in paired groups. The members of a spot pair are identified as the leading spot and the following spot. They are identified by their position in the pair in terms of the direction in which the sun rotates ( 旋转）. The number of sunspots at any one time varies. A large spot group may consist of as many as 10 groups and 300 spots across the sun. The number of spots changes in a fairly regular pattern called the sunspot cycle. The largest number occurs about every 11 years. At sunspot minimum, there are at most just a few small spots. The average lifetime of an individual spot group is roughly one solar rotation, which is about 25 days. The most persistent large spots, however, can survive for two to three months. n the last paragraph the word "persistent" means

A. important.

B. effective.

C. enduring.

D. visible.