Which one of the following is the first step in taxonomy?

In a triangle,if $r_1 = 2r_2 = 3r_3$,then $\frac{a}{b} + \frac{b}{c} + \frac{c}{a}$ is equal to

If the line $y=2x+c$ is a tangent to the circle $x^2+y^2=5$,then a value of $c$ is

Let $A = \{x \in R, x \neq 0, -4 \leq x \leq 4\}$ and $f: A \rightarrow R$ be defined by $f(x) = \frac{|x|}{x}$ for $x \in A$. Then,the range of $f$ is

$A$ wheel is rotating at $900 \ r.p.m.$ about its axis. When the power is cut-off,it comes to rest in $1 \ minute$. The angular retardation in $rad/s^2$ is

The peculiarity of sponges is