鼻腔的pH值是（）
A.<5.5
B.>8.0
C.7.1～8.0
D.5.5～7.0
E.7.0～8.0
参考答案：D
为了保证参考链路中最后一个SDH网元的定时质量得到满足，始终跟踪链上网元总数值必须是受限的，即连续跟踪的网元数不应超过（）个。
______ of the earth’s surface ______ covered with water.
A Two-third; is B Two-third; are C Two-thirds; is D Two-thirds; are
