Understanding SQL Injection Attacks

Do you know about SQL injection attacks? For example, can you answer this sample Security+ question on SQL injection attacks? Q. Attackers have attacked an online web server using a SQL injection attack. Which of the following BEST describes this? A. The attacker is attempting to overload the system with unexpected data and access memory locations. B. The attacker is attempting to impersonate a user using HTML code. C. The attacker is sending random data into a program to see if the application will crash. D. The attacker is attempting to pass commands to a back-end database server to access data. You can see the answer and explanation here.