LONDON/HONG KONG—Bitcoin strained on Tuesday to climb away from a two-week low touched a day earlier when China’s central bank reaffirmed a clampdown on cryptocurrencies and restricted trading channels for Chinese residents. The world’s largest cryptocurrency gave up most of its earlier gains and was last up 0.5 percent at $31,753. It tumbled over 10 percent on Monday, its largest one-day drop in over a month. The sell-off was sparked by the People’s Bank of China (PBOC) urging China’s largest banks and payment firms to clampdown harder on cryptocurrency trading. Crypto exchanges were effectively pushed out of China by a 2017 rule change, but over-the-counter (OTC) platforms based overseas have sprung up to receive payment from people based in China and buying cryptocurrencies on their behalf. “It basically says now OTC transactions are not legitimate … we are not allowed by the banks to transfer money for cryptocurrency purchases and …