title: Choose every legal java identifier
body: <p>I'm practicing a java quiz and failed to answer this quiz correctly.
  I thought that I checked (with checkmarks in front) all possible java identifiers but the test said that I got it wrong.</p>
  
  <p>Can you tell me what is the correct answer because the web site does not provide answer? Below there is quiz and the answers I gave.</p>
  
  <blockquote>
    <h2>Quiz of legal identifier</h2>
    
    <p>Which of the following can be used in a Java program as identifiers?
    Check all of the identifiers that are legal.</p>
  </blockquote>
  
  <ul>
  <li>☐ println </li>
  <li>☑ B4</li>
  <li>☑ sum_of_data</li>
  <li>☑ _average</li>
  <li>☐ 42isThesolution</li>
  <li>☐ "hello"</li>
  <li>☐ for</li>
  <li>☑ first-name</li>
  <li>☑ AnnualSalary</li>
  <li>☑ ABC</li>
  </ul>