题目内容：
根据以下资料，回答题：Decide which of the choices given below would best complete the passage if inserted in the corresponding blanks．
Mark the best choice for each blank on AnsWer Sheet Two．Design of all the new tools and implements is based on careful experiments with electronic instruments．First, a human gmnea pig is tested using a 31 tool． Measurements are taken 32 the amount of work done, an 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．Tiehauer． 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 (肌动描计器), 38 visible through electrical signals the work done by htmaan muscle．Another machine39 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．Tiehauer'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 ofthebody．Intime, such stress can cause a disabling disease．49 , muscle fatigue is a serious safety 50．
请在第(31)处选择正确答案_______ A．regular
B．usual
C．common
D．constant
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