Which of the following indications is the primary use for electroconvulsive therapy (ECT)

A. Severe agitation.
B. Antisocial behavior.
C. Noncompliance with treatment.
D. Major depression with psychotic features.

某公司一年购买某种货物200吨，分成若干次均匀购买，每次购买的运费为2万元，一年存储费用恰好与每次的购买吨数的数值相等(单位：万元)，要使一年的总运费与总存储费用之和最小，则应购买________次．