Please provide explanation.

An investor purchases a 100,000 bond redeemable at par that pays semi-annual coupons at an annual rate of 8% and matures in 12 years. The bond is callable at par on any coupon date beginning with the 21st coupon. If a 6% minimum annual yield compounded semi-annually is desired, the price should be determined assuming the bond is called at the time of the: (A) 20^th coupon (B) 21^st coupon (C) 22^nd coupon (D) 23^rd coupon (E) 24^th coupon