问题
选择题
.
_______ , the firemen put all the fires out quite quickly.
A．Sadly
B．Hopefully
C．Unfortunately
D．Thankfully
答案
答案：D
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.
_______ , the firemen put all the fires out quite quickly.
A．Sadly
B．Hopefully
C．Unfortunately
D．Thankfully
答案：D
=500nm的激光,用它"点焊"视网膜,每次”点焊”需要2
10-3J的能量,则每次”点焊”视网膜的时间是 s，在这段时间内发出的激光光子的数量是 。（普朗克恒量h=6.63
J·s，结果保留一位有效数字）