Minerals Ltd$.$ contracts to provide several manufacturers with tin. When a cartel of tin$-$producing countries suddenly embargoes future shipments of tin to Minerals so that it cannot fulfill its contracts, the distributor

a$.$must still supply the tin needs of its customers.

b$.$is excused from the performance of its contracts.

c$.$is liable for breach of contract.

d$.$can substitute some other material for the tin.