how to create a parent chiled dropdown in angular 2?

Question

i have a plane drop down with checkbox,like

 

(this is my app

 

https://stackblitz.com/edit/angular-xem199

) 

 

A[]

B[]

C[]

D[]

 

Now i want to change my dropdown as 

 

A[]

a[]

b[]

c[] 

 

B[]

a[]

b[]

c[]