CMFAS CGI Fire & Commercial Property Insurance question: condition of average make an under-insured claimant a co-insurer of part of the loss

Why does the condition of average make an under-insured claimant a 'co-insurer' of part of the loss?

• A Because the insurer refunds the premium on any value the insured under-declared
• B Because premium is based on the declared sum, so under-declaring underpays for the risk
• C Because average applies only where the insured asks for it when making a claim
• D Because the insured must reinsure the shortfall with a second insurer at claim time