As per Declan comment above - you get paid for what you do. With option B the Contractor is not taking the risk of errors in the (assumed) quantities.
Extra point of note is clause 60.4 that say significantly more/less (defined as more/less than 0.5% of the Prices st the Contract Date) then the extra over/under over will be assessed as a compensation event.
If you only did half the quantity it might mean that the cost per unit would have been more, and this clause allows you to reassess that (but only for the amounts outside of the 0.5% threshold)
Extra point of note is clause 60.4 that say significantly more/less (defined as more/less than 0.5% of the Prices st the Contract Date) then the extra over/under over will be assessed as a compensation event.
If you only did half the quantity it might mean that the cost per unit would have been more, and this clause allows you to reassess that (but only for the amounts outside of the 0.5% threshold)