运行下面的代码，将出现（ ）。

double hmean(double a, double b) {
    if (a == -b )
        throw runtime_error("Runtime error occurred.");
    return 2.0*a*b/(a + b);
}

int main() {
    double x = 10;
    double y = -10;

    try {
        int result = hmean(x, y);
        cout << "hmean: " << result << endl;
    }
    catch (const runtime_error& e) {
        cout << "Caught: " << e.what() << endl;
    } catch (...) {
        cout << "Caught an unknown exception." << endl;
    }
    return 0;
}

{{ select(1) }}

• 屏幕上输出 Caught: Runtime error occurred.
• 屏幕上输出 Caught an unknown exception.
• 程序调用 std::terminate()
• 编译错误