Which is true concerning union bargaining agreements, seasonal cost variations and material costs?

A) They have little effect on cost baseline because they are priced out in a lump sum format

B) They have little effect on cost baseline because these costs cannot change over the duration of the project

C) They can have major effect on cost baseline because they are time sensitive costs

D) They can have major effect on cost baseline if the proposal statement of work is open ended

Answer is given as ( C )

Can someone explain the concept and about the time sensitive costs?