我有以下HTML： @H_404_2@<div class="pricing-levels-3"> <p><strong>Which level would you like? (Select 3 Levels)</strong></p> <input class="single-checkBox"type="checkBox" name="vehicle" value="Bike">Level 1<br> <input class="single-checkBox" type="checkBox" name="vehicle" value="Bike">Level 2<br> <input class="single-checkBox" type="checkBox" name="vehicle" value="Bike">Level 3<br> <input class="single-checkBox" type="checkBox" name="vehicle" value="Bike">Level 4<br> <input class="single-checkBox" type="checkBox" name="vehicle" value="Bike">Level 5<br> <input class="single-checkBox" type="checkBox" name="vehicle" value="Bike">Level 6<br> <input class="single-checkBox" type="checkBox" name="vehicle" value="Bike">Level 7<br> </div>

实地网站：

http://www.chineselearnonline.com/amember/signup20.php

如何才能使用户只能选择其中的3个复选框？