Including the Transaction Costs of the Bid-Ask Spread,the Euro-Pound Cross $\frac { 1 } { S ^ { a } ( £ / \$ ) }$

Including the transaction costs of the bid-ask spread,the euro-pound cross exchange rate for a customer who wants to sell euro and buy pounds can be computed as

A) S^b(£/€) = S^b($/€) × S^b(£/$)
B) S^a(€/£) = S^a(€/$) × S^a($/£)
C) S^b(€/£) = S^b($/€) × $\frac { 1 } { S ^ { a } ( £ / \$ ) }$
D) all of the options