For someone learning english, why might english idiomatic expressions be difficult to understand?

here is the defaniston.

Explanation:

Idiomatic expressions are groups of words with an established meaning unrelated to the meanings of the individual words. Sometimes called an expression, an idiom can be very colorful and make a 'picture' in our minds.

i just checked their website, the mall is 1.8 million square feet.

the correct answer would be d, almost 2 million square feet.