摘要：3．C 将直观图还原得▱OABC. ∵O′D′＝O′C′＝2 cm. OD＝2O′D′＝4 cm. C′D′＝O′C′＝2 cm. ∴CD＝2 cm. OC＝ ＝＝6 cm. OA＝O′A′＝6 cm＝OC. 故原图形为菱形．

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