The simple view of the bonding in methane You will be familiar with drawing methane using dots and crosses diagrams, but it is worth looking at its structure a bit more closely. There is a serious mis-match between this structure and the modern electronic structure of carbon, 1s 2 2s 2 2p x 1 2p y 1.
Lack of sun exposure: Lack of sun exposure is a major cause of low vitamin d level especially if you live in the northern us. Less likely would be lack of vitamin d in food, but also a possibility if you are on a very restrictive diet.
Low levels of vitamin D can be attributed to a number of behaviors and conditions. Insufficient exposure to sunlight is among the top reasons vitamin D levels are low. Many parts of the world do not see sunlight for a majority of the year, limiting the opportunity for adequate exposure. In other areas, certain cultures require individuals to wear clothing that inhibits sun exposure.