An engineering team is working on the design of a new hoverboard toy for children and adults. The new hoverboard must meet all safety guidelines. It must be manufactured for $75 or less, the battery should last at least 2 hours, and the hoverboard should be able to travel at least 6 mph. The table shows the results of tests for the first three designs. Hoverboard Designs

Design Cost Speed Battery Life Safety Guidelines Met
A $70 5mph 1 hour yes
B $65 8mph 2.5 hours no
C $75 6mph 2 hours yes
Identify which design is optimal based on the criteria and constraints, and explain your reasoning.