单项选择题

He spoke English so well that I took it for granted that he was an American.

A. 他英文讲得如此好,以至我猜想他是个美国人。
B. 他英文讲得如此好,以至我想当然地以为他是个美国人。
C. 他英文讲得如此好,以至我同意他是个美国人。
D. 他英文讲得如此好,以至我承认他是美国人。