Read the passage.

changes at the end of the middle ages and the beginning of the renaissance created a demand for a more economical method of printing. the need for a fast and inexpensive way to produce written documents grew with the growth of literacy. several new ways of creating written materials were developed. one important innovation was the creation of movable metal type.

several print masters tried to perfect the use of movable metal type in order to produce books faster and on a larger scale. johannes gutenberg (c.1398-1468), was a german stonecutter and goldsmith. he was the first to develop a printing press that offered a solution for the demand of large quantities of written documents. in 1450, he formulated an alloy of lead, tin, and antinomy – a silvery, shiny grey metalloid found in nature – and created metal type out of it. gutenberg's alloy melted at a low temperature. it cast well in the die and was durable in the printing press. this meant the separate pieces of type could be used again and again. each letter was a mirror image that was carved in relief on a small block. these individual letters could be easily moved around to form words.

which feature, when included with this paragraph, would best aid a reader's comprehension?

an artist's rendering of the gutenberg printing press

the heading "gutenberg" at the top of the paragraph

a map of germany in gutenberg's time

a black and white photograph of a letter used in gutenberg's printing press