煤矿井下封闭火区是灭火的重要手段。火区封闭的原则有哪些？并做具体解释。

A、language cannot be acquired at all after the critical period
B、Cerebral plasticity after puberty is still high enough to for a successful mastery of a new language
C、the language faculty of an average human degenerates after the critical period
D、the language learning should be done as early as possible

A.74%
B.5%，产量增加了5.5%