The force of a twitch is less than that in a tetanus, BECAUSE:

There is enough elasticity in the structures transmitting force (crossbridges, thick and thin filaments, Z-disks, tendons) that the 10 nm shoening / half sarcomere produced by one cycle will not produce much force. Many cycles are needed to fully stretch these elastic elements. Ca++ must remain high as a result of a train of action potentials in a tetanus for some time to allow the full force to be developed. A is not correct because sufficient Ca++ is released after the transit of one action potential to saturate essentially all of the troponin binding sites and enable all the crossbridges to attach and cycle. Other factor(s) must explain the low twitch force. B is not correct because Ca++ reuptake is a very rapid process. Neveheless, the very high affinity of troponin for Ca++ ensures that it is saturated before the myoplasmic Ca++ falls significantly.