Please give me the answer for number 6

Seed plants that lived during the carboniferous impact life on earth today because a. they are the ancestors of all plant life on earth. b. their remains formed coal, which is an important resource. c. they produced fossils that are used to date many geologic events. d. their presence caused oxygen levels to drop, allowing animals to evolve.

People keep spending additional units of a particular resource on a want until their marginal benefit is  decreasing withgreater thanless thannot affected by  their marginal cost.

Rainbow trout is non- native to the united states. however this species is breeding with the native gula yltrout acientistrecently observed that the hola trout were becoming exitinct what is the cause of their extinction