1,3,6,10,15,?
Choose the correct choice from the following options:
A) 17
B) 18
C) 19
D) 20
E) 21
1 =1   1+2= 
<gadget id="calculator">1+2</gadget>
<output>3</output>
3   3+3= 
<gadget id="calculator">3+3</gadget>
<output>6</output>
6   6+4= 
<gadget id="calculator">6+4</gadget>
<output>10</output>
10   10+5= 
<gadget id="calculator">10+5</gadget>
<output>15</output>
15   15+6= 
<gadget id="calculator">15+6</gadget>
<output>21</output>
21   ANSW21R:21
<result>E</result>
1,3,6,10,15,?