Why are slow-twitch muscles more beneficial than fast-twitch muscles for cardiorespiratory fitness?
a. slow-twitch muscles are longer and thicker than fast-twitch muscles.
b. slow-twitch muscles need less blood to contract than fast-twitch muscles.
c. slow-twitch muscles need more recovery time between exercise sessions than fast-twitch muscles.
d. slow-twitch muscles are able to use oxygen more efficiently than fast-twitch muscles.

Answer is d) 
Slow twitch muscles are more important for cardiorespiratory fitness because they efficiently use oxygen.

fast twitch fibres are used in quick, high speed events such as sprints, in these situations there is not enough time for oxygen-rich blood to get to the muscles so fast twitch fibres do not use oxygen.

Slow twitch on the other hand, are used in endurance events like marathons when there is time for oxygen- rich blood to reach the muscles. This means that slow twitch fibres are better at using oxygen.

The efficient use of oxygen is important to cardiorespiratory fitness as it is concerned with the efficiency of the heart and lungs in transporting oxygen rich blood to muscles.