Portfolio theory can be dangerous to a small investor because:

A) he or she doesn't have much money to lose.
B) beta, the theoretical measure of risk, ignores business-specific risk, which is significant to an investor who doesn't have a large enough portfolio to diversify it away.
C) it makes investing seem more scientific than it really is.
D) the stock market is very unforgiving.

Portfolio theory emphasizes the importance of diversification, but small investors may not have a large enough portfolio to diversify away the risks associated with investing in individual companies. Beta, the measure of a stock's volatility relative to the overall market, may not capture the unique risks of individual companies (such as management or industry-specific risks) that could cause a significant loss for a small investor. Therefore, relying solely on portfolio theory could be dangerous for a small investor.