Need advise on using "article", thanks teachers!

[Help Wanted]

Hi teachers,

Would you be able to advise on the following sentences. Which is one is correct in the use of article "a" before part of China. Please advise, thanks

A. Tibet is an ancient country that is now a part of China.
B. Tibet is an ancient country that is now part of China. ( If B is the answer, then why don't we need the article "a" before part of China?)

Thanks soooo much, Help Wanted

[Casiopea]

Both are fine. The article is often omitted. I'm not sure why that is. Other like forms (adjectives of measure) require the article; e.g., a piece of, a means of, a way of, a gallon of, etc.), which might mean - and this is a guess - that our article-defiant "part of" is a matter of semantics, notably homophony (words that sound the same but have different spellings, and mean different things):

EX: . . . is now apart (separate)
EX: . . . is now a part (union)

Of course, given our context, "Tibet . . . a part of China", speakers wouldn't interpret "of" as "from" (i.e., apart from ~ separate from), but given its distribution elsewhere in the grammar, "a part" does in fact lend itself to ambiguity:

EX: They are apart. (separated)
EX: It's a part [for my car]. (a piece of equipment)

[Help Wanted]

Thank you soooo much for your advice, teacher Casiopea!