Incidentally, the use of "some" in this context is as a synonym for "approximately." Other examples:
I read that book some fifteen years ago.
we have a modifying phrase "once numbering in the millions worldwide," and this phrase needs to touch the noun it modifies.so (A) is incorrect. B) is passive and indirect and wordy, and has that awful preposition after the final comma.
"estimate" or "is estimated" is should not be the main verb.
我判断这道题,BCD有语法错误,所以排除,选项A看不出因果关系,我背景知识也不够,所以不考虑as表原因,那么as就当时间状语从句,但是1不符合主将从现的原则,2没有人说话说:谁将干啥,当谁谁将干啥的时候,所以A也排出,最后剩下E,这么推能行么?