Both U.S. EPA and its adversaries faced tough questioning this morning on the second day of arguments over the lawfulness of the agency's greenhouse gas regulations. On the one hand, the three judges of the U.S. Court of Appeals for the District of Columbia Circuit appeared to have strong concerns about whether industry challengers had standing to challenge one of the rules. But they also indicated that it might be possible for a separate challenge to existing regulations that regulate major polluters to proceed.