Business people - investors included - ask a lot of questions. In face, the phrase, "due diligence" almose implies they must ask many tough questions. They will often ask the same questions two or three time and may focus on points that you think are irrelevant.
Often, their questions will sound critical and your instinct will be to become defensive. But it's important to remain objective and not take these things personally. Focus on answering as many questions as possible until the investor is comfortable with you and your idea. Be patient and accommodating as they work through their process.
Investors must ask many tough questions and will often sound critical... don't become defensive.