Under what conditions would a particular stage in the process of succession revert back to an earlier stage ?

Consider the following statements about carbon cycle and choose how many statements are correct?

$(a)$ Atmosphere has $71\;\%$ of total global carbon
$(b)$ Decomposers contribute substantially to $CO_2$ pool by their processing of waste materials and dead organic matter of land or oceans
$(c)$ $4 × 10^{13}\; kg$ carbon is fixed through photo-synthesis annually
$(d)$ No respiratory input into atmosphere
$(e)$ Carbon constitutes $ 49\;\%$ of dry wt. of organisms

The salinity of the oceans is

Mark the correct pair

Which of the following ecosystem has the highest gross primary productivity