Concept: Reference service may include ready reference service and long-range reference service. Research queries generally belong to long-range reference service.
Step 1: Check Assertion A.
Research queries are usually complex because they may require searching many sources, evaluating documents and giving detailed information.
\[
A = \text{Correct}
\]
Step 2: Check Reason R.
For research queries, reference staff must be more actively involved. They may need to search catalogues, databases, indexes, abstracts and reference tools.
\[
R = \text{Correct}
\]
Step 3: Check explanation.
Because the reference staff has to show active involvement, research queries take longer time to respond.
Therefore, both A and R are correct and R is the correct explanation of A.