Answer
$(b)$ connecting only one appliance on this circuit might not trip the breaker again;
&
$(d)$ the appliances together had a total resistance that was too low to work with a normal 120-V circuit breaker.
Work Step by Step
Multiple appliances are usually connected in parallel. This results in reduced resistance and hence increase in current through the circuit which might go beyond the allowed limit. In that case the circuit breaker will trip and thus if one of the appliances is removed the circuit breaker might not trip again.
