Cadmium is a neutron absorber commonly used for control rods to regulate the neutron flux and control the chain reaction in reactors.

A nuclear explosion requires a mass greater than the critical mass so the chain reaction proceeds supercritically and rapidly.

Carbon-14 is produced when nitrogen-14 is struck by neutrons (from cosmic rays), converting it to carbon-14, so nitrogen by high energy neutrons is correct.

Typical reactor fuel cycles last on the order of a few years between refueling, so 'A few years' is the best choice.

Correct option selected as per the block answer_hint in the OCR header.

Amorphous means lacking a definite long-range structure; hence it means without any structure.

The block's answer hint indicates option A; energy from mass defect is given by E = Δm c^2 and the OCR answer marker corresponds to option A.

The Balmer series was identified in the 19th century; 1885 (option C) aligns with historical dates for Balmer's work.

One week = 168 h = 14 half-lives, remaining activity = 1*(1/2)^14 ≈ 6.1×10^-5 Ci ≈ 61 microCurie, closest to 60 microCurie.

The OCR shows a mark adjacent to option C and hyperons are commonly grouped into the four classes listed (Λ, Σ, Ξ, Ω).