On Sundays there were a lot of children playing in the park, parents sitting together jo

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(答案→)A. their
B. whose
C. which
D. that
解析：考查独立主格结构。句中有逗号，that不能用于非限制性定语从句，which不能作定语，排除C、D；选whose的话，从句的谓语部分只有一个sitting，不完整，排除B。当非谓语部分的主语跟主句的主语不一致时，应该带上自己的主语，这叫独立主格结构。their parents是sitting的逻辑主语。故选A。