如图，在棱长为a的正方体ABCD﹣A₁B₁C₁D₁中，P为A₁D₁的中点，Q为A₁B₁上任意一点，E，F为CD上任意两点，且EF的长为定值b，则下面的四个值中不为定值的是（   ）

DNA主要存在于（  ）

下列叙述能概括说明全球海陆所占比例的是（    ）

阅读理解What are the children doing?
    What are you doing, children? Oh, we are playing games. Look at Li Lei. He is throwing the frisby with Jim. What about the twins? They are flying kites. The kites are very nice. One like a bird, the other is like a plane. Li Tao is playing basketball with me. Han Mei and Ma Lin are singing an English song under a big tree.

生活中，一些未成年人会常常闯入“雷区”，主要原因有（    ）

①出于好奇                                          ②这些“雷区”是专门针对未成年人设下的

③社会经验不足，识别能力差          ④自制能力不强