In the following passage, some words have been deleted. Read the passage carefully and select the most appropriate option to fill in each blank.
Scientists (1) ______ Oxford University say their experimental coronavirus vaccine (2) ______ been shown in an early trial to prompt a protective immune response in hundreds of people who got the shot. British researchers first began testing the vaccine in April on about 1,000 people, half of (3) ______ got the experimental vaccine. Such early trials are usually designed only to (4) ______ safety, but in this case, experts were also looking to see what kind of immune response was (5) ______.
Select the most appropriate option to fill in blank no. 1.