To add BigInteger b1 to b2, you write  _____ .A. b1.add(b2);B. b1 = b2.add(b1);C. b2 = b1.add(b2);D. b2 = b2.add(b1);

The correct answer is A. b1.add(b2);

Here's the step by step explanation:

1. BigInteger is a class in Java that provides operations for modular arithmetic, GCD calculation, primality testing, prime generation, bit manipulation, and a few other miscellaneous operations.

2. The 'add' method is a built-in method in the BigInteger class of Java which is used to add two BigIntegers. The java.math.BigInteger.add(BigInteger val) returns a BigInteger whose value is (this + val).

3. So, if you have two BigInteger objects, b1 and b2, and you want to add them, you would use the 'add' method on one of the objects and pass the other object as an argument.

4. Therefore, to add b1 to b2, you would write b1.add(b2); This will return a BigInteger whose value is (b1 + b2).

5. The other options B, C, and D are incorrect because they are assigning the result back to one of the operands which is not necessary if you just want to add two BigIntegers.