For our middle school students, some courses are compulsory, while others are ________.

A．flexible

B．optional

C．impressive

D．significant

下列叙述正确的是                                                                                       （   ）

A．原子序数越大，元素原子半径越大

B．元素原子半径越小，越容易失去电子

C．非金属元素的氢化物越稳定，沸点越高

D．稀有气体元素的质子数越大，其单质的沸点越高