What is the main difference between the batch and at commands?

What is the main difference between the batch and at commands?
A . The batch command will run multiple times.The at command will only run once.
B . The batch command will run when system load is low. The at command runs at a specific time.
C . The at command reads commands from standard input. The batch command requires a command line argument.
D . The at command e-mails results to the user. The batch command logs results to syslog.

Answer: B