What is the DataWeave expression to access booklSBN later in the flow?

A web client submits a request to http://localhost:8081/books/0471767840. The value "0471767840" is captured by a Set Variable transformer to a variable named booklSBN.

What is the DataWeave expression to access booklSBN later in the flow?
A . booklSBN
B . attributes.booklSBN
C . flowVars.booklSBN
D . vars. booklSBN

Answer: D