有一条多肽链,分子式为C69H121O21N25S,将它彻底水解后,只得到下列四种氨基酸:分析推算可知,水解得到的氨基酸数为()
①H2N-CH2-COOH
②H2N-(CH2)5-CH(NH2)-COOH
③HS-CH2-CH(NH2)-COOH
④CH3-CH(CH3 )-CH(NH2)-COOH.
A.19个
B.20个
C.21个
D.25个