Please choose the correct ending to complete the given sentence: Health: [header] How to keep your posture straight when you carry a heavy backpack [title] Wear the backpack as it was designed to be worn. [step] Namely, wear it on your back, with all parts doing what they're supposed to do. [title] Wear both straps.

Ending1: [step] Check the side straps after every class so that you know what all the straps do. If they don't matter, just wear your backpack as it was designed to be worn. Ending2: [step] You should not just up-and-down wear it now and then. This may cause discomfort in the shoulder area or on the strap itself. Ending3: [step] Place a strap over each arm so that the straps sit evenly across each shoulder. Wearing the bag on only one shoulder causes strain and will make your posture lopsided. Ending4: [step] Wear your backpack with both straps for added comfort. When you're carrying a heavy backpack, you'll be taking your backpack off now and back on.

Answer format: ending1/ending2/ending3/ending4