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A Tree-Climbing Contest: Compare Using Unit Analysis

GMAT-4TYGTM

Rob can climb a tree in $10$ minutes while Robby can climb $5$ trees in an hour.

If Rob and Robby have a Tree-Climbing Contest that lasts $3$ hours, how many more trees will Rob climb than Robby?

A

$2$ trees

B

$3$ trees

C

$4$ trees

D

$5$ trees

E

$6$ trees