Africa's black rhino population in the mid-1970's numbered about 20,000, ten times the estimated population of 2,000 in 1997.
the estimated population
that of their estimated population
more than the population's estimated amount
more black rhinos than their population estimate
more than that of their population's estimated amount
in the mid-1970's 这个=in the mid-1970s??在这里纠结了半天判断哪个是谓语动词
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