Because a large number of electronic mail systems exists having hundreds of thousands of subscribers, prove that while Americans may write fewer letters, another method of communication has arisen to take the place of writing letters.

(a)Because a large number of electronic mail systems exists having hundreds of thousands of subscribers, prove
(b)That there exists a large number of electronic mail systems that have hundreds of thousands of subscribers is proof
(c) The existence of a large number of electronic mail systems, which have hundreds of thousands of subscribers, proves
(d) Electronic mail systems, which have hundreds of thousands of subscribers, by their existence prove
(e) The existence of electronic mail systems having hundreds of thousands of subscribers are proof