我国探明的矿产资源总量约占世界的______%．

掷3枚均匀硬币一次，求正面个数与反面个数之差X的分布列，并求其均值和方差．

根据句意，用括号中所给的词的适当形式填空。

1. Jim, don't think too much of _______, if you want people to love you. (you)

2. We should teach the young to build their ________ on hard work every day, not dream. (life)

3. Read the _______ passage as quickly as you can. (follow)

4. Though life is sometimes hard, one must face it ________. (brave)

5. — How long have you _______ skating here, Mary? (be) 

   — For about two hours.

油品计量有哪些重要性？

作为统计调查对象的个人，在重大国情国力普查活动中拒绝、阻碍统计调查的，应当（）。

A.由公安部门进行警告

B.由统计部门进行罚款

C.由工商部门责令改正，并进行罚款

D.由县级以上人民政府统计机构责令改正，予以批评教育

铁路运输支出包括哪些内容？如何理解铁路运输成本？