Design of all the new tools and implements is based on careful experiments with electronic instruments. First, a human "guinea pig" is tested using a (31) tool. Measurements are taken (32) the amount of work done, and the buildup of heat in the body. (33) joints and stretched muscles cannot (34) as well, it has been found, (35) joints and muscles in their (36) positions. The same person is then tested again, using a tool designed according to the suggestions made by Dr. Tichauer. All these tests have shown the great (37) of the new designs over the old.One of the electronic instruments used by Dr. Tichauer, the myograph (肌动描记器), (38) visible through electrical signals the work done by human muscle. Another machine (39) any dangerous features of tools, thus proving information upon (40) to base a new design. One (41) of tests made with this machine is that a tripod stepladder is more (42) and safer to use than one with four legs.This work has (43) the attention of efficiency experts and time-and-motion-study engineer, but its value goes far (44) that. Dr. Tichauer’s first (45) is for the health of the tool user. With the (46) use of the same tool all day long on (47) lines and in other jobs, even light (48) work can put a heavy stress on one small area of the body. In time, such stress can cause a disabling disease. (49) , muscle fatigue is a serious safety (50) . （48）（）

A. hand
B. labor
C. manual
D. mind