The total overhead budget of a company that uses three level analysis of variances is $140,000 per month, out of which fixed overheads amount to $100,000. The budgeted production volume is 5,000 units of output per month. The standard time per unit is 4 hours. During December 2019, the company worked 20,100 hours and produced 4,800 units. Actual factory overheads incurred during December 2019 amounted to 150,000. Calculate (i) efficiency variance, (ii) volume variance, and (iii) expense variance. Also offer a critical analysis of your answers.