According to our text, it is estimated that about % of all road collisions occur because of mechanical failures.

- 12-13%.

Explanation:

Traffic collision is one of the most common human made disaster/threat that injures or takes the life of millions of people in an year. As per the given details in the text, only 12-13% of traffic collisions are caused due to the mechanical failures while the rest 87-88% is due to the negligence or recklessness of the drivers on road(disobeying the traffic rules and overconfidence or haste). The most common cases of negligence involve disobeying the traffic rules, rash driving, uncontrolled speed-limits, driving under influence, etc. that ends up in damaging their life in most cases. The reckless attitude of the drivers is the prime cause of traffic collisions on road that not only takes their life(in most cases) but pose a threat to the others too.

the first one is d and the second a

this null effect is caused by a weak manipulation and ineffective internal validity. in a scientific study, as the one dr. morimoto is trying to perfom, internal validity refers to the confidence placed in the cause-effect relationship and to how well an experiment is done and able to eliminate confounding variables.