Which of the following has the largest population in a food chain?

Two strings $A$ and $B$ of lengths $L_A = 80 \ cm$ and $L_B = x \ cm$ respectively are used separately in a sonometer. The ratio of their densities $(d_A / d_B)$ is $0.81$. The diameter of $B$ is one-half that of $A$. If the strings have the same tension and fundamental frequency,the value of $x$ is:

When a system is taken from state $i$ to state $f$ along the path $iaf,$ it is found that $Q = 50 \, cal$ and $W = 20 \, cal$. Along the path $ibf$,$Q = 36 \, cal$. Work done along the path $ibf$ will be ........... $cal$.

The area (in square unit) of the region enclosed by the curves $y=x^2$ and $y=x^3$ is

The practical purpose of classification of living organisms is to

To distinguish between $A$ and $B$,it is observed that both elements form oxides. When burnt in air,$B$ also forms a nitride. What are $A$ and $B$?