物体做匀加速直线运动，相继经过两段距离为16m的路程，第一段用时4s，第二段用时2s，则物体的加速度是（   ）

根据句意，写出括号内所给单词的适当形式。
____ Do you think it's good for you to keep worries to  _______ (your), boys and girls?
____ In China, it's becoming much _______ (hard) for people to pass the driving tests.
____ The government has realized the importance of the _______ (safe) of the children who take a school bus.
____ More and more teachers feel worried about some _______ (teenage) strange behaviour.

小小的指纹包含了重要的信息，目前在我国居民身份证登记项目中就包括了指纹信息，那么，控制人指纹信息的遗传物质存在于细胞的（　　）