I think the correct answer would be primary source. It is an account of an event created by someone who took part in or witnessed the event. It is also known as original source. It is any document, manuscript, diary, autobiography, artifact, recording or any source that was made at the time being studied. Examples are interviews, archives, photographs, letters, films and scrapbooks. When the data from these primary sources are obtained from other source wherein they are being analyzed and interpreted then these sources will be called as a secondary source. Examples are articles, books, magazines, surveys, internet resources.
The red and the convertible cars would be considered similar i.e. they are perfect substitutes also the car and the convertible car would be the substitutes but it is not a perfect as the convertible car would be the subset of the car group plus the expenditure made on the convertible car would be high so here the elasticity is more
