Fill in the blanks with the corresponding letters: Have you ever heard of online learning? If you are unfamiliar with it, please read the following 1 introduction. Online courses are very useful 2 your goal is gaining new skills or advancing towards a cerificate. You can also use online classes for meeting professional 3 . With online classes, you choose when and where you 4 . There are no "live" classes to attend. Instead, lectures, coursework, and other activities all 5 at your convenience.