Name the gas that can readily decolourise acidified $KMnO_4$ solution.

Which of the following elements has the highest reaction rate with water?

The ratio of minimum wavelengths of Lyman and Balmer series will be

Which of the following is a non-reducing sugar?

The first life on Earth was .....

$A$ microscope consists of an objective of focal length $1.9 \ cm$ and an eyepiece of focal length $5 \ cm$. The two lenses are kept at a distance of $10.5 \ cm$. If the image is to be formed at the least distance of distinct vision $(25 \ cm)$,the distance at which the object is to be placed before the objective is: (in $cm$)