Which is the primary colonizer in primary succession on rock?

Assertion $(A):$ $A$ natural ecosystem is usually self-sustainable.
Reason $(R):$ An ecosystem having producers,decomposers,and a source of energy is self-sustainable. Select the correct answer from the options given below:

Identify the correct type of food chain.
Dead animal $\rightarrow$ Blow fly maggots $\rightarrow$ Common frog $\rightarrow$ Snake

Grassland biome for its maintenance requires

$X-$ Atmospheric input of phosphorus through rainfall is much smaller than carbon input.
$Y-$ The exchange of phosphorus between organisms and the environment is negligible.

Maximum contribution of $O_2$ is from