18. Which of the following descriptions with regard to arachidonate is right? (A) It is the substrate for the synthesis of prostaglandins by the cyclooxygenase pathway. (B) It is the substrate for the synthesis of leukotrienes by the lipoxygenase pathway. (C) Aspirin, indomethacin and ibuprofen usually inhibit cyclooxygenases by competing with arachidonate. (D) All are right. (E) All are wrong.