The correct answer is: E. None of the above
A radian is a unit of angle measurement equal to the angle subtended at the center of a circle by an arc that is equal in length to the radius of the circle. It is not a number system.
A number system is a system for representing numbers. It is a set of symbols and rules for combining those symbols to represent numbers. The most common number system is the decimal number system, which uses the symbols 0, 1, 2, 3, 4, 5, 6, 7, 8, and 9. The decimal number system is a base-10 number system, which means that each digit in a number has a value that is a power of 10. For example, in the number 123, the 1 is in the hundreds place, so its value is 100; the 2 is in the tens place, so its value is 10; and the 3 is in the ones place, so its value is 1.
The digit position value in a number system is the value of a digit in a number, depending on its position in the number. For example, in the number 123, the 1 is in the hundreds place, so its value is 100; the 2 is in the tens place, so its value is 10; and the 3 is in the ones place, so its value is 1.
The number of distinct counting digits in a number system is the number of different digits that can be used to represent numbers in the number system. For example, the decimal number system has 10 distinct counting digits: 0, 1, 2, 3, 4, 5, 6, 7, 8, and 9.
The radian of a number system is not a variable, has nothing to do with digit position value, does not equal the number of its distinct counting digits, and is not always an even number.