STRIKE, by the dozen, owing to, on a trip, for a change, etc.For EXAMPLE:All houses within 100 metres of the seas are at risk of flooding.The phrase 'at risk' means a situation involving EXPOSURE to danger.Thus the preposition 'at' will be used instead of 'in' in the 3rd part of the QUESTION. Correct Sentence: A species is considered endangered when it is at a very high risk of extinction in the world.There are some verbs/nouns/adjectives which are followed by fixed preposition given below:Exonerate from, refrain from, suspicious of, interested in, inured to, accused of, a predilection for, a RESPITE from, vexed at, etc.Example:After spending some time on the island they became inured to the hardships.