The Child Hears the Bell (Which Rings Continuously)of an Ice $\mathrm { S } ^ {D }$

Multiple Choice

The child hears the bell (which rings continuously) of an ice cream wagon, goes out to the street and buys ice cream, bites the ice cream, and salivates.The sound of the bell will become a(n) for going to the street to buy ice cream.

A) CS
B) US
C) CR
D) $\mathrm { S } ^ {D }$