The correct answer to this quiz was (c) can't. Did you get it right? When we're talking about how sure we are that something is true, we use 'must'. When we feel sure that something is NOT possible, we use 'can't'. You can find out more here: http://bit.ly/LEModalDeduction Test your grammar, vocabulary and spelling with our Johnny Grammar's Word Challenge app! Download here: https://uqr.to/ilp Neil | LearnEnglish – British Council