Reading Comprehension

In 1975 Chinese survey teams re-measured Mount
Everest, the highest of the Himalayan mountains. Like
the British in 1852, they used the age-old technique of
"carrying in" sea level: surveyors marched inland from
the coast for thousands of miles, stopping at
increments of as little as a few feet to measure their
elevation, and marking each increment with two poles.
To measure the difference in elevation between poles,
surveyors used an optical level-a telescope on a level
base-placed halfway between the poles. They sighted
each pole, reading off measurements that were then
used to calculate the change in elevation over each
increment. In sight of the peaks they used
theodolites-telescopes for measuring vertical and
horizontal angles-to determine the elevation of the
summit.
The Chinese, however, made efforts to correct for the
errors that had plagued the British. One source of
error is refraction, the bending of light beams as they
pass through air layers of different temperature and
pressure. Because light traveling down from a summit
passes through many such layers, a surveyor could
sight a mirage rather than the peak itself. To reduce
refraction errors, the Chinese team carried in sea level
to within five to twelve miles of Everest's summit,
decreasing the amount of air that light passed through
on its way to their theodolites. The Chinese also
launched weather balloons near their theodolites to
measure atmospheric temperature and pressure
changes to better estimate refraction errors. Another
hurdle is the peak's shape. When surveyors sight the
summit, there is a risk they might not all measure the
same point. In 1975 the Chinese installed the first
survey beacon on Everest, a red reflector visible
through a theodolite for ten miles, as a reference
point. One more source of error is the unevenness of
sea level. The British assumed that carrying in sea
level would extend an imaginary line from the shore
along Earth's curve to a point beneath the Himalaya.
In reality, sea level varies according to the irregular
interior of the planet. The Chinese used a gravity
meter to correct for local deviations in sea level.


Which of the following best describes the purpose of
the first sentence of the second paragraph?
A. Introduce a definition
B. Signal a transition in focus
C. Summarize the preceding paragraph
D. Draw a contrast between two different theories.
E. Present information that contradicts the preceding
paragraph.

[spoiler]B vs D

Pls post your reasons[/spoiler]