C++ quiz. Indicate lines containing errors: <pre><code class

Indicate lines containing errors:

struct A {
  mutable int a;  // 1
  const mutable int b;  // 2
  const int &c;  // 3
  mutable int &d;  // 4
  static mutable int e;  // 5
  A() : c(a), d(a) {}
};

// 1
// 2
// 3
// 4
// 5

Explanation

Line // 2 contains an error, because const and mutable specifiers are mutually exclusive.
Line // 4 contains an error, because the mutable specifier does not make sense for class member references (although almost all compilers except clang ignore this :)).
Line // 5 contains an error, because mutable specifier cannot be applied to static class members.