James has been managing a hotel construction project in a busy street. He is in the process of estimating activity durations for building walls and calculates the most likely estimate as 15 days. If pre-fabricated material is used, it would take no more than 12 days to finish the work. However, the work may be delayed and could take up to 18 days if less experienced construction workers are used to build the walls. But, the engineer in-charge of the work estimates the duration as 8 days when pre-fabricated material is used. What is the expected duration of building walls using the three-point (PERT) estimate, if the engineer's estimation is assumed correct?

A. 12 days B. 11 days C. 16 days D. 14 days