Read the following paragraph and answer the question. Give ONLY the answer, no other words or explanation.

For example:

Answer: <most likely answer, as short as possible>.

Paragraph: 
Rhesus blood type is determined by one common antigen. A person may either have the antigen (Rh + ) or lack the antigen (Rh - ).


Question: What is the common abbreviation for noting the rhesus blood type?
rh
Rh.