what is the maximum positive or negative temperature gradient that is permitted for multi-layer boards?

During the acclimatization process, what is the maximum positive or negative temperature
gradient that is permitted for multi-layer boards?

A.
Approximately 2° C (4° F) per hour

B.
Approximately 4° C (7° F) per hour

C.
Approximately 6° C (11° F) per hour

D.
Approximately 8° C (4° F) per hour