Read this and answer the question. If the question is unanswerable, say "unanswerable".

In the wake of the Coventry Blitz, there was widespread agitation from the Communist Party over the need for bomb-proof shelters. Many Londoners, in particular, took to using the Underground railway system, without authority, for shelter and sleeping through the night there until the following morning. So worried were the Government over the sudden campaign of leaflets and posters distributed by the Communist Party in Coventry and London, that the Police were sent in to seize their production facilities. The Government, up until November 1940, was opposed to the centralised organisation of shelter. Home Secretary Sir John Anderson was replaced by Morrison soon afterwards, in the wake of a Cabinet reshuffle as the dying Neville Chamberlain resigned. Morrison warned that he could not counter the Communist unrest unless provision of shelters were made. He recognised the right of the public to seize tube stations and authorised plans to improve their condition and expand them by tunnelling. Still, many British citizens, who had been members of the Labour Party, itself inert over the issue, turned to the Communist Party. The Communists attempted to blame the damage and casualties of the Coventry raid on the rich factory owners, big business and landowning interests and called for a negotiated peace. Though they failed to make a large gain in influence, the membership of the Party had doubled by June 1941. The "Communist threat" was deemed important enough for Herbert Morrison to order, with the support of the Cabinet, the stoppage of the Daily Worker and The Week; the Communist newspaper and journal.

What did Londoners use for bomb-shelters?
Underground railway system