Find All of the Real Zeros of the Function $$f ( x ) = x ^ { 3 } - 3 x ^ { 2 } - 7 x + 21$$

Multiple Choice

Find all of the real zeros of the function. Give exact values whenever possible. Identify each zero as rational or irrational.
- $$f ( x ) = x ^ { 3 } - 3 x ^ { 2 } - 7 x + 21$$

A) 3 (rational) , 7 (rational) , and $- 7$ (rational)
B) 3 (rational) , $2 \sqrt { 7 }$ (irrational) , and $- 2 \sqrt { 7 }$ (irrational)
C) 3 (rational) , $\sqrt { 7 }$ (irrational) , and $- \sqrt { 7 }$ (irrational)
D) $- 3$ (rational) , $\sqrt { 7 }$ (irrational) , and $- \sqrt { 7 }$ (irrational)