[advait_2k]

52- A
55-B
for 66 -answer choice C--for two actions in past first should be "had spent " and later action simple past which is spent.
A- the tense is used in reverse order
b-tense order incorrect
d- compares the "time spent" with "figure of 6 hours" which is incorrect
e- growing to is incorrect

Although I went with C because of Tense following ambiguity can be considered.

"an average of " is generally used to indicate average of two or more values.
In d,e this may indicate average of ...slightly less than 2 and ....3 hours(this case half hours) which can be 2.5. In this case ,it becomes ambigious either less than 2.5 hours or average of less than 2 and 0.5 hours