Meat contamination because humans do not eat air or fertilizer so meat contamination is the only logical answer.
Group 1 elements (usually called alkali metals) are not very electronegative and have small ionization energies due to that. The reason why they are not very electronegative is that they really want to loose their one valence electron so that they can have a noble gas electron configuration (completed octet).
I hope this helps.
Answer is (3) - ratio of neutrons to protons.
Isotopes are the atoms which have same atomic number but have different number of neutrons of same element.
Atomic nucleus can be stable or unstable. The stability of isotopes is based on neutron/ proton ratio. Unstable nucleus tries to become stable by emitting radiations.
