display drop down list in a node-type in drupal

Question

I have three node types

1. a-type
2. b-type
3. c-type

in my system c-type is child of b-type, and b-type is child of a-type.

b-type has one node-reference field of a-type only c-type has two node-reference field for both a-type and b-type.

In the node of c-type I want to create a select-box of all nodes of b-type which which have the same node-reference of a-type. Please let me know if you don't understand my question.