18. Although the majority of low-speed engines and many medium-speed engines are operated on heavy fuels, there is still a balance to be struck between the lower cost of the heavier fuels and the inconvenience and greater cost of fuel treatment and increased engine maintenance thatinevitably results from their use. Therefore,
(A) the use of heavy fuels could increase the cost of fuel treatment.
(B) the use of heavy fuels could reduce the maintenance duty.
(C) the inconvenience of fuel treatment is induced by the increased engine maintenance.
(D) the use of heavy fuels is not appropriate for low-speed engine but is acceptable for medium-speed engine.