What is the point the speaker is trying to make about technology development a. A mature technology requires less innovation. b. Technical evolution is close to maturation in certain fields. c. New types of energy are expected to mature in the near future. d. Nanomaterials or photonics and carbon nanotubes are environmentally friendly.

排球基本技术中最难掌握的技术，并且在排球比赛中最积极、最有效的进攻手段是（）。

A.吊球

B.发球

C.扣球

D.拦网