All of the following are reasons why engraving can be difficult, except which? a. engraving requires a lot of pressure to be applied to the tool as it glides across the metal surface and the time it takes to complete. b. many artists struggle with the sound the tool makes while etching on metal. c. before beginning the engraving, the artist must apply a thin layer of acid to the metal surface on which the engraving will be done. d. when an artist creates an engraving, he must plan it carefully because mistakes cannot be corrected.