Should I use a warning icon or a question mark icon in a Windows message box?

Many know of the MessageBoxIcon of type "question". If you are not particularly familiar with this icon, it is just a glorified question mark. I am curious as to whether or not this icon is acceptable in professional applications. For example, let's assume that I have a button which, when clicked, will clear all text fields on the entire form. When the button is clicked I would like to warn the user about what his action is about to do. I could write either of the following:

MessageBox.Show("Really clear all data?", "Clear confirmation", MessageBoxButtons.YesNo, MessageBoxIcon.Question);

OR

MessageBox.Show("Really clear all data?", "Clear confirmation", MessageBoxButtons.YesNo, MessageBoxIcon.Warning);

Which would you all say is the more professional of the two?