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Answer:d)introns; exons
Explanation:Introns are the non-coding portion of the DNA. They do not code for a gene that can be expressed hence the need for them to be removed from the coding genes(Exons). The exons are then joined together after the introns have been removed to create a sequence of amino acids.
The answer is; Physical weathering by water would carve out the face of the cliff.
The bottom of the cliff would register the greatest weathering because of the force of the water hitting the rocks in the fall. As a cave is curved from the base of the cliff, the overhanging part of the cliff eventually collapses hence retracting the cliff a small distance upstream of the river. The diagram below demonstrates this;