Here we have a Must Be True question. The correct answer will be 100% supported by the passage.
We are told that when a computer vandal gains access to one computer in a business with multiple, linked computer systems that use identical operating system software on all of its computers, the vandal will have automatically have access to the data on all the computers.
However, if such a business introduced minor variations into its operating system software, unauthorized access to all the computers AT THE SAME TIME could be virtually eliminated.
So, to reiterate, the stimulus does not state that introducing minor variations into a business's operating system software will no longer allow a vandal to have access to the data on any of the computers, merely that it will no longer allow a vandal to have access to the data on all of the computers AT THE SAME TIME.
This is exactly what answer choice (C) is saying: "If a business introduces a variety into its linked computer operating systems, it will have increased overall protection for its systems, but will not have protected every computer from viral invasion."
We know that the variety will help ward against a vandal gaining access to all the computers at once through one computer, but it won't ward against the vandal gaining access to at least one computer.
Hope that clears things up! Please let us know if you have any other questions.
