Solution:
'Since' refers to the starting point of an action. It relates here and is the correct answer, as it's given that mother got ill in the last month and she is still ill.

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Solution:
The correct preposition to be used here is 'in'. In cases like this, the 'field' is a place or location, so the preposition is 'in'. For example - He works in a bakery. When the object in the sentence is an object (a thing), then the preposition would be 'on'. For example - He works on computers.

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Solution:
The most common use of 'between' is when something is in the middle of two things or two groups of things. It is sometimes used in the phrase in between. The most common use for 'among' is when something is in or with a group of a few, several, or many things.