[单项选择]The post-war explosion in the use of detergents (清洁剂) was a prime cause for the deteriorating (恶化) condition of Thames twenty-five years ago. Between 1951 and 1961, detergent use increased three times. A tragic example of the results of this was when a man drowned because, though help was at hand, the would-be rescuers could not see him through the mass of foam.
Previously, detergents had been of vegetable origin and caused no trouble. The new "hard" (technically, non-biodegradable 不能分解的) detergents contained elements which could not be degraded in the treatment plants sewage (污水) works. These detergents decreased the efficiency of plants by an estimated 30 percent. So when the waste water from the works was sent into the river it still contained much "hard" detergent, which foamed or spread on the surface and greatly reduced the amount of oxygen naturally taken into the water. These detergents were also poisonous to fish.
The threat became so serious that in 1957 the Standin
A. the similarity of specialists to doctors
B. the comparison between the river and the patient
C. how healthy the Thames is
D. the ways specialists monitor the Thames

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