For 1

C) It cannot be calculated with sufficient accuracy to justify the requirement of environmental impact studies prior to plant construction.

C suggest that once we constructed the plant we can calculate the environmental impact while the whole passage didn't suggest anything such as that is why C is wrong and A is correct and can safely by deducted from the passage.

For 2

E) point out an exception to a general claim

The use of word "exception" make this option wrong, read the bold text again with full concentration, it says:

Estimates for these emissions are relatively straightforward to make and show that the consequences are small compared to the benefits of generating greenhouse gas-free hydroelectric power.

This is not an exception to what is stated in the passage except that this bold text. This bold text present a related information it didn't suggest that "Estimates for these emissions are relatively straightforward to calculate the environmental impact rather it says straightforward to show that the consequences are small.

That is why E is wrong here.