F=ma, so a=F/m. Thus, the more massive object accelerates more slowly, that is, the 21 kg object is three times slower than the 7 kg one (2nd variant, 7kg is 3 times faster than 21kg)
b. The 7 kg object accelerates three times faster than the 21 kg object. ;
Applying the same net force to two objects with different masses results in different accelerations. The 7 kg object accelerates three times faster than the 21 kg object. Therefore, the correct answer is B.
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