'Challenging' is a synonym for 'difficult', indicating something that requires effort to overcome.

'Diligent' means having or showing care and conscientiousness in one's work or duties.

'Frugal' means sparing or economical with regard to money or food.

'Hasty' means done with excessive speed; quick.

'Meticulous' means showing great attention to detail, which is synonymous with 'thorough'.

'Guide' is a synonym for 'navigate'.

'Vigilant' means keeping careful watch for possible danger or difficulties.

'Vivid' means producing powerful feelings or strong, clear images in the mind, which is synonymous with 'bright'.

'Pessimistic' means having a negative outlook, which is the opposite of 'optimistic'.

'Ephemeral' means lasting for a very short time, similar to 'transitory'.

'Seperate' is spelled incorrectly; the correct spelling is 'separate'.

'Elated' means very happy or proud; therefore, the synonym is 'joyful'.

'Meticulous' means showing great attention to detail, making 'thorough' the correct synonym.

'Descend' means to go down, which is the opposite of 'ascend'.

'Simple' is the antonym of 'difficult', indicating ease.

'Contract' means to become smaller, which is the opposite of 'expand'.

'Rare' is the antonym of 'frequent', indicating something that does not happen often.

'Selfish' is the antonym of 'generous', indicating a lack of willingness to share.

'Heavy' is the opposite of 'light'.

'Quiet' is the antonym of 'noisy', meaning making little or no sound.

'Pessimistic' is the antonym of 'optimistic', which refers to a tendency to see the worst aspect of things.

'Complicated' is the opposite of 'simple'.

'Failure' is the opposite of 'success', indicating a lack of achievement.

'Fish' is an aquatic animal, while the others are terrestrial.

'Car' is not an animal, while the others are all pets.

'Plentiful' means existing in large quantities, which is synonymous with 'abundant'.

'Reduce' means to make smaller or less, which is synonymous with 'diminish'.

'Hasten' means to speed up, which is the same as expedite.

'Mitigate' means to make something less severe, serious, or painful.

The Berlin Wall fell on November 9, 1989, marking a significant point in the Cold War.