The most frequent cause of failure of a cast crown restoration.. lack of attention to tooth shape, position, and contacts

The most frequent cause of failure of a cast crown restoration is:

a- failure to extend the crown preparation adequately into the gingival sulcus

b- lack of attention in carving occlusal anatomy of the tooth

c- lack of attention to tooth shape, position, and contacts***

d- lack of prominent cusps, deep sulcus, and marginal ridges.