TOEFL Listening: TPO-TOEFL阅读TPO - WG3IJ0V23R3580IEN$

According to paragraph 5, which of the following is true of melanic forms in Liverpool? A. Increased sulphur dioxide pollution decreased their numbers after 1975. B. Their population slowly increased from 1975 to 1989. C. From 1950 on, they have seldom been seen in the Liverpool area. D. Before 1975 their population was higher than that of light-colored moths.