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  3. We’ve all used thermometers to measure temperature but sometimes we need to measure temperature in places you can’t put

We’ve all used thermometers to measure temperature but sometimes we need to measure temperature in places you can’t put

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问题     We’ve all used thermometers to measure temperature but
sometimes we need to measure temperature in places you can’t put           【S1】______
a thermometer — for example, inside a rocket engine. One of the
best methods for those really hard-to-get-to place is based on laser       【S2】______
spectroscopy. Light from a laser pointing at the region of interest        【S3】______
is absorbed by molecules and some of it are re-emitted back                【S4】______
towards the source where it can be measured. The re-emitted light
is shifted in frequency (changed in wavelength) by amounts that
depend on the molecule and also the temperature and pressure of            【S5】______
the surrounding gas because of collisions among the molecules.
The single, sharp frequency from the laser is thus smeared out in
to a whole spectrum of frequency whose shape can be used to                【S6】______
determine the temperature provided so that the molecular
transitions and line shapes are known.  Because it ’is the main           【S7】______
component of the air, it is convenient to use nitrogen since its
molecular properties are known from room temperature to                    【S8】______
temperature of about 1,200 degree Celsius from experimental
measurements. Although this is a large range, it is far short of           【S9】______
what it is needed for some applications.                                   【S10】______
【S4】
选项
答案fire→is
解析 根据上下文，主语some of it中的代词it应该指light。
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