We live in the computer age. People use computers to do much of their work. But few people know how to look after them. Remember the following when you use your computer:
    ① Keep your computer in a cool ,dry room. Too much or too little heat is bad for computers. Damp (潮湿) is also bad for them.
    ② Do not let people smoke near your computer. Smoke of all kinds is very bad for many parts of a computer.
    ③ Do not eat or drink when you are near your computer. Pieces of food and a little water in the keyboard are also bad for a computer.

对于用天平、量筒测某种液体的密度来说，其实验设计有如下二种方案：

（1）一、二两种方案，选第____ 种方案进行实验，实验误差最小。
（2）用你选择的方案进行实验，所测密度的表达式ρ_液体＝____ 。
（3）经过思考小明将另一方案稍加改进，提高了测量结果的准确性。改进后的步骤C分为两步，请你帮他写出改进内容：
第一步：____ 
第二步：____ 
（4）用步骤中的字母，写出方案改进后小明所测液体密度的表达式ρ＇_液体＝____