The basic direction and trend of social and historical development is forward or upward, but its development is not smooth sailing, but a spiral or wave-like process. The fundamental reason for this vacillation and repetition is that the old social forces and the reactionary class are unwilling to take the initiative to withdraw from the historical stage, and they want to carry out tenacious and lasting resistance with the progressive forces. Therefore, the answer to this question B. The stubborn and persistent resistance of the old social forces and the reactionary class.

Satisfied, please adopt.