Three years ago the average age of a class of six members was 21 years. A boy have been added, the average age of the class is the same today. What is the age of the boy? Choose the correct option:
A) A)1
B) B)2
C) C)3
D) D)4
E) E)5
6 * 24= 
<gadget id="calculator">6 * 24</gadget>
<output>144</output>
144   7 * 21= 
<gadget id="calculator">7 * 21</gadget>
<output>147</output>
147   `3   3
<result>C</result>
Three years ago the average age of a class of six members was 21 years. A boy have been added, the average age of the class is the same today. What is the age of the boy?