The supply chain manager at D Dax conducts an audit to assess the quality of one of the imported components in their manufacturing process. As per the records the imported components are normally distributed, with mean of 116 cm and standard deviation of 2.6cm.

1. Find the probability that one randomly selected unit has a length greater than 121cm.
2. Find the probability that each of the three randomly selected units have lengths exceeding 121cm.
3. Find the probability that if three units are randomly selected, their mean length exceeds 121cm.
4. Explain the differences between parts (a), (b) and (c).