Reduced new effect duration stacking

[Scol]

How does reduced new effect duration from different sources stack? If I have 30% reduction from Out of Phase, 30% from Restless Bones and 40% from Time Shield, will I have:
a.) 40% reduction - only highest one apply
b.) 100% reduction - they stack additively
c.) ~70% reduction - they stack multiplicatively

[edge2054]

C

[Scol]

Thx