新分泌釉质基质所含矿物质仅占矿化物总量的()。
A.15%~20%
B.25%~30%
C.35%~40%
D.45%~50%
E.55%~60%
参考答案:B
——Now lots of teenagers spend a lot of time playing computer games.
——Yes. Teachers and their parents should prevent them ______________ too much.
A.to play
B.play
C.plays
D.playing
根据冲击地压事故严重程度、可控性、救灾难度和影响范围,将事故分为几级?