Network neutrality means that, to the extent permitted by law, all Internet users can access network content, operate applications, access devices, and select service providers according to their choices. This principle requires network operators to treat all Internet content and access equally. To prevent it from controlling the priority of data transmission in the interests of commerce, and to ensure the "neutrality" of network data transmission. According to the above definition, which of the following violates the principle of net neutrality? ['A mailbox has a contract with a network operator, so it is faster to use the mailbox when the network is congested.', 'A network operator uses technical means to block websites with fraudulent nature.', 'Some pictures of a website are only accessible to registered users.', 'A network operator collects broadband usage fees from subscribers.']