Why carbon-12 was set as the standard of atomic mass?

In 1961, IUPAC announced that the standard atomic mass is changing from oxygen=16. to carbon-12=12.

1. For physicists, carbon-12 has been used as one of the standards in mass spectroscopy.
2. For chemists, if change the standard from the average mass of oxygen to oxygen-16, the atomic mass has to change 275ppm. However, using carbon-12 as 12., it only has to change 42 ppm.
3. There are many organic compound in the nature, such as hydrocarbon compounds and protein.
4. Carbon-12 is abundant and stable.
5. The atomic mass of hydrogen, the lightest atom, is still large than 1.

1. 碳12為質譜儀測量時的其中一個標準
2. 若以O-16為16，則化學家的平均原子量必須改變275ppm；而以C-12為12，則只要改變42ppm
3. 許多有機化合物中含有碳而沒有氧
4. 含量多且穩定：同位素:含量 = C-12：98.93% = O-16：99.76%；同位素:半生期 = C-14：5700年 = O-15 : 122秒
5. 質量最小的氫仍大於1

ref:
http://www.sizes.com/units/atomic_mass_unit.htm
https://www.scienceforums.net/forum/showthread.php?p=395022
http://www.iupac.org/publications/ci/2004/2601/1_holden.html
http://zhidao.baidu.com/question/22147666.html?si=6
http://chntlb.chshs.tpc.edu.tw/science/content/1984/00030171/0008.htm
http://presolar.wustl.edu/ref/IsotopeAbundanceTable.pdf
http://en.wikipedia.org/wiki/Oxygen-16
http://en.wikipedia.org/wiki/Isotopes_of_carbon