Discussion
Assertion (A) : Headward erosion of stream leads to river capture.
Reason (R) : Elongation of a stream in source region causes headward erosion.
Which one of the following is correct ?
Answer: A
The key is Both (A) and (R) are true and (R) is the correct explanation of (A).
Headward (backward) erosion is the cutting back of a stream at its source, so the stream is lengthened up its own valley. When this lengthening carries the head of one stream through the divide into the valley of a neighbour, the neighbour's upper course is diverted into the more actively eroding stream. That is river capture, or piracy, so (A) is true. (R) is true too: the elongation of a stream in its source region is what headward erosion does.
The weak point is (R). Elongation at the source is nearly the definition of headward erosion rather than a separate cause, so a reader could call (R) a restatement and prefer 'true but not the explanation'. No source found settles that choice, so the answer follows the key: the lengthening at the source is the mechanism that brings a stream to the divide and into position to capture another. Options C and D are wrong because neither statement is false.
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