We have been told:<br/><br/>‘which' and ‘it' are used to replace a noun, not a sentence or idea; ‘so' is used to replace a verb/idea in a sentence. <br/><br/>Based on the above, is the following sentence correct:<br/><br/>Playing football is good for health because it makes your heart healthy.<br/><br/>In the above sentence, 'it' would refer to 'Playing football'. So, is this usage of 'it' correct?<br/><br/>Thanks.