Please choose the correct ending to complete the given sentence: Sports and Fitness: [header] How to do sit ups [title] Start by lying on your back with your knees bent. [step] Sit ups work best if you do them on a soft surface, such as a mattress. Keep your knees bent at a 90-degree angle, with your feet flat on the floor.

Ending1: [title] Push your legs up until your thighs are directly over your hips. [step] Then, push your legs up so they are straight up and over your head, as high as they will go comfortably. Ending2: [title] Cross your right leg over your left foot. [step] Sit up straight, with your knees slightly bent. Ending3: [substeps] Avoid bending your knees very much or you may fall backwards. You should also extend your arms as far as you can, which can strain your spine. Ending4: [substeps] You may be more comfortable if you lie on an exercise mat while you're doing sit ups. [title] Put your fingertips on the back of your ears.

Answer format: ending1/ending2/ending3/ending4