Meteor showers and individual streaks of light that flash across the sky every night are generated when tiny flecks of celestial detritus, often no larger than grains of sand or of pebbles, burn up speeding through the atmosphere.

A. grains of sand or of pebbles, burn up speeding
B. grains of sand or pebbles, burn up while speeding
C. grains of sand or pebbles, which burn up while speeding
D. a grain of sand or pebble, which burns up as it speeds
E. a grain of sand or a pebble, burns up when it speeds

A: no larger than grains...OF pebbles
Here, of pebbles conveys the following incorrect meaning:
no larger than GRAINS OF PEBBLES.
Since grain and pebble both imply something VERY SMALL, grains of pebbles is redundant.
Eliminate A.

C: pebbles, which burn up...
Here, which seems to refer to pebbles, implying that PEBBLES burn up while speeding through the atmosphere.
The intended meaning is that FLECKS OF CELESTIAL DETRITUS burn up while speeding through the atmosphere.
Eliminate C.

D: a pebble, which burns up...
Here, which seems to refer to a pebble, implying that a PEBBLE burns up as it speeds through the atmosphere.
The intended meaning is that FLECKS OF CELESTIAL DETRITUS burn up while speeding through the atmosphere.
Eliminate D.

E: flecks of celestial detritus...burns up
Here, burns (singular) does not agree with flecks (plural).
Eliminate E.

The correct answer is B.

Hey!
I have a serious problem!
I solved this question with correct explanations in 55 seconds, but in the actual Gmatprep test I couldn't understand the problem at all.
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Mukherjee