While Cox, 25, took the gold, Britain’s newly crowned most decorated female paralympian of all time Dame Sarah Storey finished in fourth place in the same C4-5 event.
Speaking to broadcaster Channel 4 after the race, Cox said: “I’m over the moon.”
She added: "I can't believe it. I'm over the moon. I have only been doing it a year and a half and being on top of the world is amazing."
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It was another golden day at the Rio Paralympics for Team GB with Hannah Cockroft retaining her T34 100m title and Andy Lewis claiming gold in the PT2 Para-triathlon.
A handful of silver and bronze medals were also won by the Brits, with Kare Adenegan taking silver in the T34 100m, Sabine Fortune a bronze in the women’s F20 shotput, and Alice Tai a bronze in the S10 100m backstroke.
Great Britain now has 35 medals – 15 of which are gold, putting them in second place on the table.
Aside from Tai’s bronze, there was disappointment at the aquatics centre, however. 15-year-old Ellie Robinson who finished fourth, and Ellie Simmonds – Robinson’s Paralympic inspiration – who finished sixth in the S6 50m freestyle.
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