The Test Cases Derived from use cases?

(A) Are most useful in uncovering defects in the process flows during the testing use of the
system.
(B) Are most useful in uncovering defects in the process flows during real world use of the
system.
(C) Are most useful in covering the defects in the process flows during real world use of the
system.
(D) Are most useful in covering the defects at the Integration Level.