A. The parent can pass arguments to the child when called, but the child can only pass values back as new artifacts in the event.

B. The parent must create a new artifact in the event named return_xxx, and the child must return values by creating artifacts with the naming convention arg_xxx.

C. The parent can pass arguments to the child when called, and the child can return values from the end block.

D. The parent must create a new artifact in the event named arg_xxx, and the child must return values by creating artifacts with the naming convention return_xxx.