How many isoprene units are present in abscisic acid?

Chemically,'digestion' is:

Which of the following represents a nucleoside?

Which of the following statement$(s)$ is(are) true?
$(1)$ Oxidation of glucose with bromine water gives gluconic acid.
$(2)$ The two six-membered cyclic hemiacetal forms of $D-(+)-$glucose are called anomers.
$(3)$ Hydrolysis of sucrose gives dextrorotatory glucose and laevorotatory fructose.
$(4)$ Monosaccharides cannot be hydrolysed to give polyhydroxy aldehydes and ketones.

The number of six-membered and five-membered rings present in norethindrone (a synthetic progesterone) are respectively:

Assertion : $Mg$ is not present in the enamel of human teeth.
Reason : $Mg$ is an essential element for the biological functions of humans.