<i>a) I can't make it sorry, because I'm working all day.</i><br/><br/>b) <i>I can't make it sorry, am working all day.</i><br/><br/>Could you argue that 'am working all day' is a second verb phrase in b?<br/><br/><i>c) I can't make it sorry, working all day.</i><br/><br/>I don't think this could be argued as ok.