= Solution
\b[Sample B should show two FCC peak families], one from a copper-rich phase and one from a silver-rich phase. Their lattice parameters are set by the compositions of the coexisting phases, not by one <lattice parameter> at the overall alloy composition.
For corresponding <Miller indices> the silver-rich phase has a larger <lattice parameter> and hence a larger interplanar spacing, so its reflection occurs at smaller $2\theta$. Some reflections may overlap or be weak. Relative intensities depend on phase fractions, atomic scattering factors, absorption and texture; a raw peak-height ratio is not directly a <mass fraction>.
At room <temperature> the mutual solubilities in the supplied Cu–Ag diagram are small, so the two families would be near the elemental copper and silver positions. Slow cooling is important: a metastable supersaturated single phase would be a different interpretation. Quantitative phase or composition analysis would require calibrated <diffraction> and an appropriate refinement, not merely counting the visible peaks.
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