Please choose the correct ending to complete the given sentence: Sports and Fitness: [header] How to do a jack knife sit up [title] Lie flat on the floor with your arms and legs extended. [step] Your arms should be lifted so that they're above your head. Make sure your spine is in a neutral position so your lower back isn't arched or rounded.

Ending1: [title] Twist your upper body slightly to the right. [step] With this turn, you're on your knees instead of face up, but still tilted slightly downwards. Ending2: [substeps] If you'll be exercising on a hard floor, lay down an exercise mat before you begin. [title] Take a deep breath before you begin. Ending3: Allow your knees to extend behind you, over the center of your back, and allow them to drop below your ribcage. [substeps] Refrain from dropping the weights too much or the corners of your back may collapse. Ending4: [title] Flip your upper body over using a leg extended behind you on the opposite side of your body. [step] Your knees should be bent and the lower back should be hanging off of the floor.

Answer format: ending1/ending2/ending3/ending4