If we are constantly taking in more and more C-14, why does its concentration in us not increase?

the answer is definitely c.

the answer for sodium is 0.186

the answer for rubidium is 0.251

the answer would be a

on passing the small ampere of electric current through the tube made of ar and hg of cfl generates uv rays which excites the fluorescent and thus emits the high radiation by consumption of just 5- 12 w in comparison to incandescent light which requires more power such as around 40 - 55 w to lighten.

moreover the life span of fluorescent or cfl is around 8000 h whereas the life span of incandescent is around 1200 h

hence, the building installed with incandescent lights bears more electricity bill.