Not until he failed the exam _______he had left his books untouched .

A．did he realize

B．had he realized

C．he realized

D．he had realized

G系列smartaudio声音系统对用户有哪些利益（）.

A.在网上聊天或网络电话时，减低噪音和回音消除，让语音更清晰

B.在看大片时可以自动启动宽频音域，音响更震撼

C.多种音效模式让音乐更动听，满足顾客对不同音乐的欣赏

D.3D的环绕声效果可以在玩游戏时身临其境