To support the inference that honeybees—not bumblebees—are responsible for infecting the studied flowers, the following two pieces of evidence from the text are most relevant:
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"All of the honeybees researchers collected were infected with both viruses." - This suggests that honeybees are carriers of the viruses.
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"More than 90 percent of bumblebees collected close to apiaries carried black queen cell virus." - While this indicates that bumblebees near hives have the virus, it implies a connection to the proximity of commercial hives and their infected honeybee populations, thereby hinting that honeybees could be the source of the infection.
These pieces of evidence highlight the connection between honeybee presence and the spread of viruses affecting bumblebees and potentially flowers.