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Hope this answer isn't too late...
For Question 1, seeing as you are taking an average, you can you the formula for summing an arithmetic progression.
This is given by:
n/2(2a + (n-1)d)
Where n is the number of terms, 999 in this case
a is the first term, d is the difference in the progression.
Thus the average is then:
1/2(2a + (n-1)d)
So, the answers are:
A 500
B 1000
C 999
D 2000
Note that C is different to B as a=1, n=999, d = 2 for C so the above expression is:
1/2(2 + 998*2) = 1 + 998 = 999
whereas a = 2, n = 999, d = 2 for B
Perhaps someone can double check this, but I reckon it is right
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