Which of the following is incorrect with respect to $Chondrichthyes$?

Which of the following statements is incorrect for $Nostoc$ and $Anabaena$?

The proposition $(p$ $\Rightarrow \;\sim p) \wedge (\sim p$ $\Rightarrow p)$ is a

The resistance of a wire is $R$. If the length of the wire is doubled by stretching,then the new resistance will be

$A$ number $n$ is chosen at random from $S=\{1, 2, 3, \ldots, 50\}$. Let $A=\{n \in S: n+\frac{50}{n} > 27\}$,$B=\{n \in S: n \text{ is a prime}\}$ and $C=\{n \in S: n \text{ is a square}\}$. Then,the correct order of their probabilities is

The frequency of vibration in a vibration magnetometer of the combination of two bar magnets of magnetic moments $M_1$ and $M_2$ is $6 ~Hz$ when like poles are tied together and it is $2 ~Hz$ when the unlike poles are tied together,then the ratio $M_1: M_2$ is