answer: C
Consider c as 1, then c = 1 and 2c = 2: 0 integer is between 1 and 2, Thus C is correct.
Consider c as 2, then c = 2 and 2c = 4: 1 integer is between 2 and 4 (3), Thus C is correct.
*We know that 2c = c + c. All the values between c and 2c are c+1, c+2, …, 2c-1. So there are (2c-1)-(c+1)+1 = c-1 integers between c and 2c.

Very nicely explained. Thanks.

Recall that the number of integers between two integers a and b, inclusive, is b - a + 1. However, here we want the numbers of integers between c and 2c, excluding themselves. Therefore, the number of integers is 2c - c + 1 - 2 = c - 1. Note that we subtracted 2 on the left side of the equation because we needed to exclude the endpoint values c and 2c.

Alternate Solution:

We can express the integers between c and 2c as c + 1, c + 2, … , c + (c - 1). Since c + 1 is the first integer, c + 2 is the second integer and so on, c + (c - 1) is the c - 1st integer.

Answer: C

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