The issues that had to be dealt with when restoring Leonardo's last supper included all of the above: the experimental method used by Leonardo, the work of earlier restorers, changing levels of heat and humidity, and <span>a bomb falling on the monastery.
Leonardo was known for his experimental style of painting, many other artists tried to restore his painting, heat and humidity also had an effect on the painting, and in 1943, a bomb fell on </span><span>Santa Maria delle Grazie, where Last Supper was located, almost destroying the painting in the process.</span>
I think A is the answer
hope this helped :)
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