If (x + k ) is the common factor of x2 + ax + b and x2 + cx + d , then what is k equal to ?

• A. $$ {(d-b) over {(c-a)}}$$
• B. $$ {(d-b) over {(a-c)}}$$
• C. $$ {(d+b) over {(c+a)}}$$
• D. $$ {(d-b) over {(c+a)}}$$