The constructors in the Rational class allow initialization of the Rational objects in several different ways. Which of the following will cause an error?
a. rational r1 = new Rational();
b. Rational r2 = r1;
c. Rational r3 = new Rational(2, -3);
d. Rational r4 = new Rational (3.5);
e. Rational r5 = new Rational (10);