Select the most appropriate words to fill in the blanks.

Today computer viruses spread ______ a dizzying speed ______ way of file downloads.

at; by

Today computer viruses spread at a dizzying speed by way of file downloads.

Explanation:

• By; by: This combination is not suitable here. "By way of" is a common phrase used to indicate the method or means, and "by" is used to express agency or the means by which something happens.

• At; from: This combination is not the most appropriate. "At" is used to express a point in time or location, which doesn't fit the context of spreading viruses. "From" is used to indicate the origin, but it doesn't match the intended meaning.

• At; by: This is the correct combination. "At" is used here to express the speed at which viruses spread, and "by" is used to indicate the means or method of spreading.

• In; from: This combination doesn't fit the context. "In" is not the right preposition to describe the speed of spreading, and "from" is not the correct preposition to indicate the method.