Which letter will replace the question mark (?) in the

Milk tastes sour when kept in the open for sometimes due to the formation of-

The element used for making solar cells is

The noble gas used in radiotherapy is

The gas used for filling weather  balloons is-

The element used in the manufacture  of safety matches is —

Which metal is extracted from seawater?

The nucleus of an atom consists of-

The gas used for filling weather balloons is-

Bronze is an alloy of-

The pH of a neutral solution is —