A company has decided to implement an electronic signature scheme based on public key infrastructure. The user's private key will be stored on the computer's hard drive and protected by a password. The MOST significant risk of this approach is: 
A、impersonation of a user by substitution of the user's public key with another person's public key. 
B、forgery by using another user's private key to sign a message with an electronic signature. 
C、use of the user's electronic signature by another person if the password is compromised. 
D、forgery by substitution of another person's private key on the computer.