​The diagram below depicts the generalized distributional curves (A to D) of allochthonous organic matter and autochthonous production by different autotrophic groups, as a stream transitions to a river.​

The following are sources of organic matter:

i. Allochthonous
ii. Autochthonous from phytoplankton
iii. Autochthonous from bottom attached algae
iv. Autochthonous from aquatic macrophytes

Choose the correct option that matches the distributional curves (A to D) to the sources (i to iv):

Correct option is A

Explanation-

Curve A: Starts high in smaller streams and declines toward larger rivers
Allochthonous input (i) — dominant in small streams due to terrestrial leaf litter.

Curve B: Low in small streams but rises gradually and dominates in large rivers
Autochthonous from phytoplankton (ii) — these flourish in larger rivers with deeper, slower waters and light penetration.

Curve C: Peaks at intermediate stream size, with moderate contribution
Autochthonous from aquatic macrophytes (iv) — rooted plants that thrive in medium-sized streams.

Curve D: Present mostly in smaller streams, declines later
Autochthonous from bottom-attached algae (iii) — these attach to rocks and are more common in small, shallow, fast-flowing streams.

So, the correct answer is option a - A-i, B-ii, C-iv, D-iii