Questions

If some action has started in the past and is still continuing, then present perfect continuous tense should be used instead of present continuous tense. So, the phrase ‘am waiting’ needs to be replaced with ‘have been waiting’ to make the sentence contextually correct. Thus, the correct formation would be, ‘I have been waiting for three –quarters of an hour’.

‘Regret for’ needs to be replaced with ‘regret’ to make the sentence grammatically correct. This is because ‘regret’ is not followed by a preposition. Thus, the correct formation would be, ‘I regret using objectionable words against a man so mighty’.

When water freezes its density decreases.

Water at ordinary temperatures contracts and increases in density as it is cooled, like most substances. But at about 4°C it reaches a maximum density and then decreases in density as it approaches the freezing point.

Blueberries are the saddest fruits.

Reason ::

In English some colors are associated with feelings. Blue is associated with sadness. When you "feel blue" it means that you are feeling sad.

Hence, as blueberries has blue in its name it indicates that that is the saddest fruit in logic.