1404-23：In para.16(line 149),“So much of well-to-do America now lives antiseptically in enclaves, tranquil and luxurious, that shut out the world”,句中“that”是先行词“enclaves”所在的定语从句（that shut out the world）的引导词，可是“that”前有逗号，该定语从句应该是非限定性定语从句，应该用“which”引导，为何原文却用“that”来引导？