In "The Red-Headed League," why does Sherlock Holmes beat upon the pavement outside of Jabez Wilson's office?

A.
He wants to find out how strong his walking stick is in case he needs to protect himself.

B.
He wants to find out where John Clay is hiding in Saxe-Coburg Square.

C.
He wants to find out if the ground under the pavement has been dug out.

D.
He wants to find out whether Wilson can hear him inside the office.