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 a vitamin can result from eating a poor diet or not being able to absorb the vitamins you consume. Older adults, vegetarians and people with digestive disorders such as celiac disease or Crohn's disease may have trouble getting enough B-12. Sometimes a vitamin B-12 deficiency occurs for unknown reasons.