[greenyoda]

If you respond to them, you might ask them which specific statements in your article they claim are untrue.

However, you may want to consult a lawyer, since they may sue you for defamation if you don't take the article down, and you want to be aware of what the risks are. For example, even if your article's claims are justified, they can probably afford to pay lawyers much more than you can, so you may run out of money before you can even argue your case in court.