Please choose the correct ending to complete the given sentence: Work World: [header] How to join the air force quickly [title] Meet age requirements. [step] To join the air force for the first time, you must be at least 18, and able to be enrolled in basic military training before your 28th birthday. If you plan to apply for officer training school, you must be able to be commissioned before your 35th birthday.

Ending1: You must be under the age of 16 to apply for air force teaching positions. [substeps] If you do not have a full three years of college education that qualifies you for air force teaching positions, you may not know about flying at all. Ending2: If you cannot talk to your parents about this matter, your decision to enlist might result in the death of your parents. [substeps] You'll need a letter or postcard from your previous military service experience, along with many physical gear and trip photos. Ending3: If you plan to apply for military academy as an adult, you must be a six year-old at the time of your 20th birthday. If you want to apply for any other military training, you must pass both medical and behavioral screening. Ending4: If you want to apply to be a physician, nurse or allied health, you must be in commissioned officer training before your 40th birthday. [substeps] If you are already in the military, these age requirements may vary.

Answer format: ending1/ending2/ending3/ending4