Which option identifies a validity concern in the following scenario? A researcher contacted possible research subjects about whether they would be interested in participating in an observational study. They would have to report to a facility once a week for three months, and either asked to complete a survey or simply observed walking on a treadmill, or both. The researcher asked them to mail back a postcard indicating their intent to participate. A Subjects should always know when they will be surveyed or observed on the treadmill.
B Declaring one’s intent to participate in the study is not the same as explicitly giving permission.
C The fact that the subjects can select themselves adds reporting bias.
D Needing to report to a specific location adds exclusion bias since it might be too far away for some subjects.

Tom is having a problem with his washing machine. he notices that the machine vibrates violently at a frequency of 1500 rpm due to an unknown rotating unbalance. the machine is mounted on 4 springs each having a stiffness of 10 kn/m. tom wishes to add an undamped vibration absorber attached by a spring under the machine the machine working frequency ranges between 800 rpm to 2000 rpm and its total mass while loaded is assumed to be 80 kg a) what should be the mass of the absorber added to the machine so that the natural frequency falls outside the working range? b) after a first trial of an absorber using a mass of 35 kg, the amplitude of the oscillation was found to be 10 cm. what is the value of the rotating unbalance? c) using me-3.5 kg.m, find the optimal absorber (by minimizing its mass). what would be the amplitude of the oscillation of the absorber?