A. their B. whose
C. which D. that
【易错】容易误选B，认为这是非限制性定语从句。
【分析】最佳答案是A。与上面一题相似，their parents seated together joking 不是非限制性定语从句，而是一个独立主格结构，因为空格后的动词 seated 不是谓语，而是一个过去分词，因为 seat 作动词用时，是及物动词。比较以下相似题：
(1) On Sundays there were a lot of children playing in the park, _______ parents were seated together joking.
A. their B. whose
C. which D. that
选B。whose parents were seated together joking 为非限制性定语从句，因为其后有完整的谓语 were seated。
(2) On Sundays there were a lot of children playing in the park, and _______ parents were seated together joking.
A. their B. whose
C. which D. that
选A。因为句中有并列连词and，整个句子为并列句。
(3) On Sundays there were a lot of children playing in the park, _______ parents sitting together joking.
A. their B. whose
C. which D. that
选A。their parents sitting together joking 为独立主格结构。
(4) On Sundays there were a lot of children playing in the park, _______ parents sat together joking.
A. their B. whose
C. which D. that
选B。whose parents sat together joking 为非限制性定语从句，因为句中有完整的谓语 sat。
(5) On Sundays there were a lot of children playing in the park, _______ parents were sitting together joking.
A. their B. whose
C. which D. that
选B。whose parents were sitting together joking 为非限制性定语从句，因为句中有完整的谓语 were sitting。
■ If the man is only interested in your looks, _____ just shows how shallow he is.
A. as B. which
C. what D. that
【易错】容易误选A或B，误这是非限制性定语从句。
【分析】正确答案为D，注意逗号前是一个条件状语从句，逗号后是该状语从句的主句，that 在此代表前文所述的情况，用作主句的主语。类似地，下面几道题也选 that，而不选 which：
(1) If you promise to go with us, _____ will be OK.
A. as B. which
C. and it D. that
(2) If you want a double room, _____ will cost another ￡15.
A. as B. which
C. what D. that
(3) Whether you go or not, _______ is quite all right with me.
A. that B. which
C. and it D. so
(4) When I say two hours, _____ includes time for eating.
A. as B. which
C. what D. that