If a - b = 4 and a2 + b2 = 30, find the value of ab.  Choose the correct choice from the following answers:
A) A)7
B) B)12
C) C)15
D) D)18
E) E)20
Explanation:   2ab = (a2 + b2) - (a - b)2   = 30 - 16= 
<gadget id="calculator">30 - 16</gadget>
<output>14</output>
14   ab= 7   7nswer: 7
<result>A</result>
If a - b = 4 and a2 + b2 = 30, find the value of ab.