The answer is b because the pattern of these terms r that odd numbers r positive and even numbers are negative. 50 is even so it is negative, while 51 is odd so it is positive. Remember that negative numbers are below zero. If u add something to a negative number it is like subtracting positive numbers. -51+50 is -1. Because if 51 was positive it would be 51 minus 50. The answer is b. If u dont understand or if it is wrong tell me. :D