【正确答案】正确答案：When two sounds never occur in the same environment, they are in complementary distribution. For example, the aspirated English stops never occur after (s), and the un-aspirated ones never occur initially. Allophones of the same phoneme are usually in complementary distribution. The allophones of/p/, for instance, are also in complementary distribution. The unaspirated(p =) occurs after /s/, while the aspirated (ph) occurs in all other environments except after /s/.