A reaction – bonded silicon nitride ceramic has a strength of 300 MPa and a fracture toughness of 3.6MPa 
. with y = 1 in the fracture toughness equation, what is the largest size of internal crack that this material can withstand without fracturing?
(a) 91.6 µm (b) 82.3 µm
(c) 74.6 µm (d) 45.8 µm