研究人员常用超声波和光来探测距离．探测海底深度时应用______，探测宇宙天体间距时常用______．

I shouldn’t let it bother me. It really doesn’t matter anyway. But it does bother me ! All those people are preparing to (21) a new century, a new millennium (千年) as well. And it just isn’t (22) !
Most of us schedule our lives by the Gregorian (23) . Our years are measured from the (24) of Christ. The first year of the first century is 1, or 1 A.D. The last year of that century is the year 100. Simple Yes! A century is 100 (25) long. A millennium is 1,000 years long, which is (26) ten centuries.
Since the last year of the first century is the year 100, the first year of the (27) century is 101. Follow that pattern, century by century and you’ll get my (28) . The year 2000 is the last year of the twentieth century. After all, (29) thousand means twenty hundreds. Twenty hundred years means twenty centuries. The year 2000 is the (30) , the final, the one that’s still here, year of the twentieth century!!!!
I know, I’m getting too (31) . But it isn’t fair, While most people will really be enjoying this coming (32) celebration, those of us in the know will be yawning. Everyone else will be (33) for the new millennium and we’ll be saying, "It’s not until next year."
I don’t agree with the (34) , "Ignorance is bliss." But in this case, perhaps it’s (35) .

A．hours

B．days

C．months

D．years