text: Scraping a mineral sample on a porcelain plate is used to test the streak of the material.  Some minerals leave a streak of a different colour than you might expect from the colour of the sample itself.  For example, many iron compounds are greyish/steely in colour, but leave a reddish streak.  This can be used as an identifying characteristic.  The porcelain plate can also be used as part of a hardness test, in that a very hard mineral (i.e. diamond) would scratch the plate without really leaving a streak.\n\nAs far as the second question:  C - it describes a quality (the relative size of the sun and Earth), but not a quantity (as the others do), thus it is a qualitative statement.