A$.$ Because those transporting gold and silver coins were often robbed $-$ so paper certificates were issued.

B$.$ Because gold and silver coins were difficult to transport $-$ so paper certificates were issued.

C$.$ Because gold and silver coins no longer had any real value $-$ so paper certificates were issued.

D$.$ A and $B$ only.

Why was paper currency needed?

A$.$ Paper currency lowered the cost of transactions.

B$.$ Paper currency lowered the costs of production.

C$.$ Paper currency lowered transportation costs,

D$.$ All of the above.