Lee manufacturing uses a standard cost system with overhead applied based on direct labor hours. The manufacturing budget for the production of 5,000 units for the month of June included 10,000 hours of direct labor at \($15\) per hour, \($150,000\). During June, 4,500 units were produced, using 9,600 direct labor hours, incurring \($39,360\) of variable overhead, and showing a variable overhead efficiency variance of \($2,400\) unfavorable. The standard variable overhead rate per direct labor hour was

a.   \($3.85.\)

b. \($4.00\).

c. \($4.10.\)

d.  \($6.00.\)