What is common between parrot,Platypus and kangaroo?

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:

If $a$ and $b$ are unit vectors such that $(a + 2b)$ and $(5a - 4b)$ are perpendicular to each other,then the angle between $a$ and $b$ is ................... $^o$.

Three bodies,a ring,a solid cylinder,and a solid sphere,roll down the same inclined plane without slipping. They start from rest. The radii of the bodies are identical. Which of the bodies reaches the ground with maximum velocity?

In the symbiotic association between fungi and plants (mycorrhiza),which nutrient does the fungus absorb from the soil and provide to the plant?

$A$ certain mass of a gas occupies a volume of $2 \ L$ at $STP$. To what temperature must the gas be heated to double its volume,keeping the pressure constant?