Take the task of installing the light bulbs and break it into stages.

Stage 1: Choose a light bulb for the first lamp
There are three colors to choose from, we can complete stage 1 in 3 ways

Stage 2: Choose a light bulb for the second lamp
Since we already used one of the light bulbs in stage 1, there are only two light bulbs remaining.
So, we can complete stage 2 in 2 ways

Stage 3: Choose a light bulb for the third lamp
Since we've already used 2 of the 3 light bulbs in stages 1 and 2, there's only 1 light bulb remaining.
So, we can complete stage 3 in 1 ways

By the Fundamental Counting Principle (FCP), we can complete all 3 stages (and thus install all three light bulbs) in (3)(2)(1) ways (= 6 ways)

Answer: C

Note: the FCP can be used to solve the MAJORITY of counting questions on the GRE. So, be sure to learn it.

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