Three bells commence tolling together and toll at intervals of 3,6,9 seconds respectively. In 60 minutes, how many times do they toll together?  Choose the correct choice from the following answers:
A) 4	B) 10	C) 15	D) 21	E) 17
L.C.M of 3,6,9 is 18. i.e after each 3 min they will toll together.So in 60 min they will toll 20 times.As they have initially tolled once,The answer will be  20+1= 
<gadget id="calculator"> 20+1</gadget>
<output>21</output>
21.   ANSWER:21
<result>D</result>
Three bells commence tolling together and toll at intervals of 3,6,9 seconds respectively. In 60 minutes, how many times do they toll together?