Identifying radiation by absorption properties

Steps:

• Subtract background (28 cpm) from readings: no sheet gives 722 net cpm; thin card gives 450 net cpm.
• Thin card reduces counts by over 37%, indicating alpha particles, which are stopped by paper-thin materials.
• Remaining 450 net cpm after card shows penetrating radiation, consistent with beta particles passing through card.
• 5 mm aluminum (implied to reduce to near background in standard setup) stops beta particles, leaving no significant gamma rays, which penetrate aluminum.

Why D is correct:

• Alpha stopped by thin card; beta penetrates card but absorbed by 5 mm aluminum; no gamma as aluminum blocks remaining radiation (inverse square law irrelevant here).

Why the others are wrong:

• A includes gamma, but aluminum absorption implies none.
• B excludes beta, but post-card counts too high for alpha/gamma only.
• C excludes alpha, but card absorption too strong for beta/gamma only.

Final answer: D