With respect to the variance analysis of materials when there are several direct material inputs that can be substitued for each other to at least some degree, which of the following statements is NOT true?

a) The quantity variance is caused by making more or less units of good product than planned in the master budget.

b) The yield variance is caused by using more or less inputs in total than the standard total inputs allowed.

c) The mix variance represents the cost variance resulting from using materials in a non-standard proportion.

d) The price variance is the result of paying higher or lower prices than budgeted for the various direct materials.