7. Trey and his mother were observing the passing trains by the railway. Starting from the locomotive, 6 carriages passed in the first 10 seconds, and the total time for the train to pass was 2 minutes and 45 seconds. Assuming the train was traveling at a constant speed, the train had approximately ( ) carriages.
(A) 60
(B) 80
(C) 100
(D) 120
(E) 140
7. C.

Notice that, 2 minutes 45 seconds $=165$ seconds. According to the problem, the number of carriages is $165 \times \frac{6}{10}=99$. Therefore, the train has approximately 100 carriages.