We must keep a positive attitude toward the future ________ we may encounter some difficulties.[ ]

A. however  

B. as if

C. even if  

D. since

现有一把弹簧测力计和一个带绳套的小的空矿泉水瓶、足量的水及煤油．用这些器材和物质测定煤油的密度．

（1）依次写出测量步骤和该步骤中被测物理量的符号：                            ．

（2）用上述测得的物理量计算煤油的密度                          ．