Can I practice test questions by topic?

Yes you can.  Each test type (Car, Motorcycle, etc) offers options to take a quick quiz, a full mock test, or to practice questions by topic.  When you select Practice questions by topic, a list of all available topics is displayed.  Click to select the topic you want to study and you'll see options to take a short test for that topic, a full test, or you can answer questions in a topic singly.  Each question you answer correctly in a topic is notated with a check mark, so you can easily track your progress.