Article Q: elected as/appointed as + no article + position

Ex1:

a. Obama was elected as president in 2008.
b. Obama was elected as the president in 2008.
c. Obama was elected as a president in 2008.

Ex2: J

a. Jackson was appointed as chairman in 2010.
b. Jackson was appointed as the chariman in 2010.
c. Jackson was appointed as a chairman in 2010.

My understanding is that (A) is the most used form while (B) and (C) are also grammatically correct.

My question: In the case of (A), why is the article "the" or even "a" missing? Why is it grammatically acceptable to put no article there?


Thanks in advance.

The most common version is:

Obama was elected president in 2008. Jackson was appointed chairman in 2010.

The three you have are all used, but frankly sound a little odd unless in context.

I think the best explanation is that in conversation rhythm must be maintained, and so there exists a limited amount of flexibility beyond the strict grammatical prescriptions.