Hi Lucy,
first of all look at the topic meaning clarity because you can have the OE
meaning-clarity-sc-question-bank-by-carcass-souvik-145694.htmlMmmmm I guess is time to overhaul the book or at least on certain key points.
Back to your question these are the main difference
MAY / MIGHT usage
Probability
Uncertain Possibility
Formal allowance but in spoken english we use can, in general
Cheers to someone
SHOULDIt is used basically with hypothetical situation in which we need to convey a certain condition
OR 99% for the GMAT purpose it expresses advice
You should study harder to overcome this test
So in this scenario we have a probability to find a cure in the next few years. So should is incorrect
I am quite sure, really convinced that this is a tricky question and overall our set on meaning is quite good.
This set forces you to tackle a question from the meaning standing point even if you do not know the specific rule. If you analize the sentence very carefully all choices but A has a sense.
Hope this helps