There was a very interesting remark in a book by an Englishman that I read recently ________what he thought was a reason for this American characteristic.
句意：最近我读到一位英国人的书，书中有句很有趣的话，他认为这就是造成美国人性格的原因。there be sb. /sth. doing属于固定结构，意为“有人或物做了某事/处于某种状态”。因此，本题的正确答案为A。