______ is the substitution of an agreeable or inoffensive expression for one that may offend or suggest something unpleasant. For instance, we refer to "die" as" pass away".

A．Financial problems.
B．Boredom.
C．Fatigue.
D．Lack of insurance schemes.

A．it avoids the need for subjecting a laboratory subject to repeated major surgery
B．it permits scientists to monitor changes frequently
C．the transplanted cells grow as they would in their normal site
D．the transplanted cells can be easily grown in vitro