Step 1: This question is from "Me and the Ecology Bit." It refers to Jim's conversation with Mr. Johnson.
Step 2: Jim advises Mr. Johnson about his habit of driving his car even for very short trips, like two blocks to the post office.
Step 3: Jim points out that this practice wastes fuel and causes pollution. By walking, Mr. Johnson could save 'petrol' (gas) and also reduce his contribution to air pollution. Saving fuel and reducing pollution are ways to save the 'ecology' (B), but the direct resource being saved is petrol.