St Pertersburg, is a very beautiful city, was once called Leningrad..A. whichB. thatC. whe

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(答案→)A. which
B. that
C. when
D. where
解析：考查非限制性定语从句。本题的先行词是St Pertersburg，在非限制性定语从句指,which指代先行词，做为定语从句的主语。That不能引导非限制性定语从句；when和where在定语从句中做状语。故A正确。