The knowledge point to be investigated in analyzing this question is the scope of application of the agent. The scope of application of agency includes: (1) acting as an agent for various legal acts. This is the most common agency behavior. For example, signing contracts and fulfilling obligations on behalf of others. (2) Acting as an agent to perform certain financial and administrative acts. Such as patent application, trademark registration, tax payment, legal person establishment, change and cancellation of registration. (3) Acting as an agent in civil litigation. Scope of agency inapplicability: (1) Legal acts of personal nature. Such as making a will, dissolving a marriage, etc. (2) Legal acts agreed by both parties that should be carried out by themselves. For example, a processing contract with a designated processor, a performance contract with a designated actor, etc. (3) acts that the principal has no right to carry out. Item B: Marriage is a personal act and cannot represent. Therefore, item B is correct. Item C: Performance is an act that should be performed by me according to the agreement of both parties, and I can't act as an agent. Therefore, item C is correct. Item A: You can apply for a patent as an agent. Therefore, item A is wrong. Item D: Signing a sales contract is the most common agent. Therefore, item D is wrong. To sum up, the correct answer to this question is BC.