Passage: National Highway 10 (NH 10; formerly NH 31A) links Siliguri to Gangtok. Sikkim National Transport runs bus and truck services. Privately run bus, tourist taxi, and jeep services operate throughout Sikkim and also connect it to Siliguri. A branch of the highway from Melli connects western Sikkim. Towns in southern and western Sikkim are connected to the hill stations of Kalimpong and Darjeeling in northern West Bengal. The state is furthermore connected to Tibet by the mountain pass of Nathu La.

Question: What was the highway that links Siliguri to Gangtok previously known as?
NH 31A