The official answer is A. idiom "more than " is used for comparion and when numbers are present. confused between usage of "greater than' and "more than". :banghead::banghead:
Guys please validate
"The gyrfalcon" is singular subject so pronoun following it must be singular hence C using "their" and D using "they" are out.
E is awkward as in second clause adverbial phrase beginning with "now" seems to be independent of first and therefore a semicolon must be used instead of comma. Hence eliminated.
Between A and B.
As "more than " and "greater than" both are correct idioms. The elimination of one of them depends on which one is appropriate with respect to present question.
Here the comparison is between the number of gyrfalcon birds now and in 1970, which is better implies by the comparison with "greater than". Hence eliminate B.
Therefore I would go with A