Add Like Terms, If Possible, and Write the Result in Descending

Add like terms, if possible, and write the result in descending powers of the variable.
-6m⁷ - 4m⁷ - 2m⁷ + 3m⁷

A) 3m²⁸
B) 21m
C) 3m⁷
D) Cannot _{be simplified}