4. A bank has three assets. It has $75 million invested in consumer loans with a three-year duration, $39 million invested in T-bonds with a 15-year duration, and $19 million in six-month maturity T-bills. What is the duration of the bank's asset portfolio in years?

A) 3.95 years B) 6.16 years C) 6.50 years D) 7.38 years E) 11.51 years