54. Which of the following is an indivisible obligation?*

(A) To sing the National Anthem in the next Pacquiao fight.

(B) To construct a pavement measuring 2 meters wide and 10 meters long.

(C) To lecture for 5 Saturdays in a tax seminar.

(D) To pay a loan obligation of P100,000 in 10 equal monthly instalments.

Brief explanation will do, thanks!