Read the following passage and mark the letter A, B, C, or D on your answer sheet to indicate the correct answer to each of the questions from 30 to 34 

     As computer use becomes more common, the need for security is more common than ever. One of the greatest security threats in the online world is computer hacking.  

     Computer hacking is the unauthorized access to a computer or network of computers. Hackers are people who illegally enter systems. They may alter or delete information, steal private information, or spread viruses that can damage or destroy files. But how exactly can a hacker get into a system to do these things?  

     Most hackers use information called protocols that are built into computer software. These protocols allow computer to interact with one another. Protocols are sort of like computer police officers. When a computer connects to another system, the protocols check to see if the access is valid. The protocols can also determine how much information can be shared between the two systems. Hackers can manipulate the protocols to get unlimited access to a computer system. 

     In fact, just the act of entering a computer network is considered hacking. This is commonly called passive hacking. Passive hackers get a rush from just being able to access a challenging system like a bank or military network. Another kind of hacker tries to do damage to a system. After hacking into systems, these hackers release viruses or alter, delete, or take information. Known as active hackers, they are, by far, the more dangerous of the two.  

          The easiest way to protect a system is with a good password. Long and unusual passwords are harder for hackers to guess. For even greater security, some online services now use “password-plus” systems. In this case, users first put in a password and then put in a second code that changes after the user accesses the site. Users either have special cards or devices that show them the new code to use the next time. Even if a hacker steals the password, they won’t have the code. Or if the hacker somehow gets the code, they still don’t know the password

What does “plus” in “password-plus” probably mean?