Describe why direct materials rate variance and direct materials efficiency variance may be computed with reference to different points in time. A. Direct materials rate variances are often computed before purchase, while direct materials efficiency variances are often computed after purchase. Purchasing managers are typically responsible for efficiency variances, while production managers are typically responsible for rate variances. B. Direct materials rate variances are often computed before the usage of the materials, while direct materials efficiency variances are often computed after their usage. Both purchasing and production managers are responsible for usage variances. C. Direct materials rate variances are often computed at the time of purchase, while direct materials efficiency variances are often computed at the time of usage. Purchasing managers are typically responsible for rate variances, while production managers are typically responsible for usage variances. OD. Direct materials rate variances are often computed at the time of usage, while direct materials efficiency variances are often computed after the usage. Production managers are typically responsible for rate variances, while purchasing managers are typically responsible for usage variances.