(b) That's wrong. There are various contracts that involve 'bailment'. Bailment is when a person has possession of another's goods and undertakes to deal with them according to the owner's instructions. These instructions may be to store the goods, repair them, transport them or clean them.
If the contract contains no agreed terms regarding the bailee's liability, the common law makes it a term of the contract that the bailee will exercise reasonable care, as judged in the circumstances. This means that they will be liable for any damage caused by a failure to take such care.
In this case, B has failed to take reasonable precautions to keep the sacks safe, and would be in breach of the duty.