He expressed his gratitude to his head teacher, without _______-help he wouldn’t have made such great progress in his studies.

A．whose

B．his

C．whom

D．which

20世纪20年代后期和30年代前中期，中 * * 党内先后出现了三次“左”倾错误，试分析其错误观点及一再出现的原因。