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.
B选项的解释是什么鬼。。。。老师的翻译不就是选题方式吗,怎么就不对了。。。还是看CD的讨论和RON的解释才让人恍然大悟。。。