The code that can be used to add the cost of an umbrella to the total cost will be $\_\_\_\_\_\_\_\_\_.$

a$.$

if self.ui$.$umbrella.AddToChecked$()==$True:

cost$=$cost$+20$

b$.$

if self.ui$.$umbrella.Checked$()==$True:

cost$=$cost$+10$

c$.$

if self.ui$.$umbrella.Add$()==$True:

cost$=$cost$+20$

d$.$

if self.ui$.$umbrella.isChecked$()==$True:

cost$=$cost$+20$