Radioactive decay percentage is independent of initial mass Steps:

• Half-life of 2.5 minutes means the mass halves (50% decrease) in that time, regardless of starting amount.
• A 70% decrease in 2.5 minutes implies 30% remaining, but exponential decay law gives exactly 50% remaining after one half-life.
• The stated 70% decrease contradicts the half-life, as percentage decayed depends only on time relative to half-life, not initial mass.
• No numerical initial mass can be determined, as the scenario is physically inconsistent and lacks absolute mass values.

Why B is correct:

• Not applicable; no option fits due to inconsistency.

Why the others are wrong:

• A: Same issue; percentage decay fixed by time and half-life.
• C: Same issue; percentage decay fixed by time and half-life.
• D: Same issue; percentage decay fixed by time and half-life.

Final answer: Not enough information.