Write the Sentence as an Equation or Inequality $21 - x = 36$

Multiple Choice

Write the sentence as an equation or inequality. Use x to represent any unknown number.
-The sum of 21 and a number is 36.

A) $21 - x = 36$
B) $36 + 21 = x$
C) $21 = 36 + x$
D) $21 + x = 36$