Answer:
the variable balance is declared as a local variable and is different from the instance variable balance.
Step-by-step explanation:
In this code, the deposit method is not working as intended because the variable balance is declared as a local variable and is different from the instance variable balance. This is occurring due to the keyword double being placed before the balance variable within the deposit method. This is basically creating a new variable called balance that is only targetable by the local scope within the deposit method. Removing this keyword would instead target the class instance variable balance which is what is needed. In this case, the balance local variable being created is always null and would cause the method to not function.