Answer taken from the block's provided answer key (header).

With equal initial masses, the ratio B/A = 4 = 2^(t/18) (from difference of decay exponents), so t/18 = 2 ⇒ t = 36 years.

The block's OCR answer key indicates option C as the marked lifetime value for a metastable electron state.

Diffraction of electrons by crystals demonstrates their wave-like behavior (de Broglie hypothesis).

The OCR answer key indicates Paschen series as the match for the given upper limit wavelength.

One hour ≈ 60 min ≈ 60/7.6 ≈ 7.9 half-lives, so remaining fraction ≈ (1/2)^8 ≈ 1/256, closest given option is 1/250.

Answer taken from the block's provided answer key.

Number per second = current / e = 3.2×10^-3 A / 1.6×10^-19 C ≈ 2×10^16 electrons/s, matching option A from the answer key.

The potential required to lift an electron to an excited state is referred to as the excitation potential.

Using λ = h/(mv) and v = 0.01c for an electron gives λ ≈ 2.42 Å, so option B is correct.