An experiment was conducted to determine the effects of the age of newsprint on the brightness (in percentage) of the recycled paper. Four aging periods were considered, along with five concentrations of sodium hydroxide (NaOH), used to de-ink the paper. The effect of sodium hydroxide's concentration is not of interest. The blocking variable is

where are the choice options?

colorless gas, exhibiting a blue glow when placed in an electric field

b) solid

its a type of rock

intermolecular forces are the forces between molecules or particles.

there are two kinds of van der waals forces: weak london dispersion forces and stronger dipole-dipole forces.

in molecule of water there are dipole-dipole forces between negatively charged oxygen from one molecule of water and positively charger hydrogen of another molecule of water.

hydrogen bond is an electrostatic attraction between two polar groups that occurs when a hydrogen atom (h), covalently bound to a highly electronegative atom such as flourine (f), oxygen (o) and nitrogen (n) atoms.