在公式ρ= 中，当质量m一定时，密度与体积V之间的函数关系可用图象表示为（　　）

已知{f_n（x）}满足f₁（x）= $\frac{x}{\sqrt{1+x^2}}$ （x＞0），f_n+1（x）=f₁f_n（x），

Always forgetting your long passwords(密码)? Worried about being hacked(被攻击)?All these worries will soon become history. Scientists have found that upper-casing(大写)your password can help you reduce the chance being hacked.
Usually passwords are six letters or more.However,50 percent of people choose a common word or simple key combination for their password. The most used passwords are 123456, abcdef and abc123. In fact, if your passwords are in lower-case(小写),it will be easily hacked by a hacker. According to a study, it only takes a hacker ten minutes to hack a six-letter password which is in lower-case, but if you make the letter in upper-case, it can take up to ten hours. So if you want to make your account much safer, you should make your password upper-case. Of course, you can also add numbers or symbols to your password. Most of the time a hacker's computer has to work for 18 days to break a password, but if you choose a nine-letter password that includes numbers and symbols, it can take a hacher's  computer 44,530 years to break it.

分析复杂的电路中各用电器的连接关系时，为了将电路简化，通常先把电路中的电流表和电压表进行简化，正确的处理方法是（  ）