A shorter form of question "Did it happen by coincidence?"

Question

Imagine one person saying:

"Oh, both Mary and John called me at the same time".

Another asks:

"Did it happen by coincidence?".

I want to find the shortest possible way of asking the same question. I found the following options, but not sure if they sound correct or naturally:

1. Did it happen by coincidence?
2. Was it by coincidence?
3. Was it a coincidence?
4. Was it by chance?
5. Was it a chance?

I feel (3) is OK. Is there any even shorter form?

EDIT: THE RESULTS:

As of 28th May 2015 there are the following question suggestions:

• 6 vote(s) - Coincidence?
• 4 vote(s) - By chance?
• 4 vote(s) - Was it a coincidence?
• 1 vote(s) - Chance?
• 1 vote(s) - Randomly?
• 1 vote(s) - Incidentally?
• 1 vote(s) - Synchronicity?
• 1 vote(s) - By coincidence?
• 1 vote(s) - Was it by chance?
• 1 vote(s) - Was it coincidence?
• 1 vote(s) - Was it coincidental?
• 1 vote(s) - Was it by coincidence?
• 0 vote(s) - Was it a chance?
• 0 vote(s) - Did it happen by coincidence?

and remarks:

• 1 vote(s) - Jinx!
• 1 vote(s) - GMTA!
• 1 vote(s) - Wow, that's random.

As a vote I considered direct answers, mentions and upvotes, even in comments. This suggestion is both popular and very short:

By chance?

I'm choosing choster's answer, as it includes the winner and is the most helpful. Thank you everyone for your input!

Answer 1

Younger or slangier Americans might ask

Randomly?

or remark

Wow, that's random.

Random in this case does not mean mathematically random, but unusual, unexpected, or inexplicable. To ask, Randomly? is to ask for confirmation that the simultaneous calls or callers were not planned or expected, and that it happened simply by chance.

This use of random, as with the use of any versatile word favored by a younger generation, has its detractors; however, an OED editor noted in a radio interview that

"The specifically mathematical sense we have only from the late 19th century,"

[NPR All Things Considered].

For an older or more formal audience, asking By coincidence? or By chance? would be better understood, or if you want to be explicit, I wold endorse your options #1, #3, or #4.

Answer 2

[person 1] "Oh, both Mary and John called me at the same time".
[person 2] "Coincidence?"

That's all. You can use just the word itself which is a single sentence all by itself.

Answer 3

It is correct and natural in common vernacular to say something similar to:

"Oh, both Mary and John called me at the same time."

"By chance?"

"Yes!"

From your options, numbers 2, 3, and 4 are all normal depending on the region. Thus, you could replace "By chance?" in my example above with "By coincidence?" or "Was it a coincidence?" and it would sound correct and natural.

Answer 4

I think the natural reply that is shorter is

was it coincidence

the 'a' is not really necessary.

perhaps this is bad english and it should be

was it coincidental

but I also like very much the short 'By chance' but people would say - was it coincidence - more than - was it a coincidence - IMHO...