Why Might Pickets Prevent Trucks and Railcars from Entering a Struck

Why might pickets prevent trucks and railcars from entering a struck business?

A) because picketing during a strike carries a legal obligation on the part of the employer to cease operations, including delivery of goods and materials
B) because unions often refuse to cross another union's picket line
C) because the union has a legal right to obstruct the business of the employer the union is striking against
D) because operators of trucks and railcars must get permission from the picketing union to enter the premises