How many years did it take the black death to spread from sarai to cairo?

The Black Death (Bubonic Plague) followed the Trade Routes. The Trade routes provided access to all corners of the known world. The increased use of the trade routes ensured that the disease spread throughout the World. We should also remember that it was not just Europe and Africa that were devastated by the deadly disease. Countries such as China suffered horrendously from the 1328 outbreak with their population dropping from 125 million to 90 million during just the middle half of 14th century.
hope it helped:)

The answer to your question would be b