Auto fill & auto blank cell

hi

i create the Bill of material sheet using multiple column dependent data validation (drop down list)

My selection is

when i select cell Value in Column A drop down list then cell Value in Column B drop down list show only range/List related with column A cell value.

My problem is

When I change or delete the Column A cell value then Column B value is as per last selection value until i Chang or delete it


I want solution for
when i change or delete the column A value then Column B cell value must be blank until i select it from drop down list.

Thanks in advance for your help