Why is a cap trade system better policy to reduce carbon emissions then a carbon tax

1)

i))the process of preparing food by green plants in the presence of sunlight is called photosynthesis.

ii) leaf is the site for photosynthesis.

iii) it takes place as,

6h2o+6co2=c6h12o6+602

for a different answer set the answer would be because the oxygen in the water had depleted (apex)