Answer
a.
Work Step by Step
Ancestors of the horsetails and club mosses, which are members of the seedless vascular plant group, flourished during the Carboniferous period. These plants were gigantic; some standing close to 40 meters tall.
These tall trees thrived when the ocean waters receded but became submerged when the seas rushed in again. Sediment then buried these submerged trees. Even more sediment settled upon previous layers, squeezing out the residual water left in the trees that were dead but preserved underneath the layers of sediment. Pressure and heat caused the remains to eventually turn into coal.
