以下关于C++模板结构体的代码片段中，哪一段可以正确编译并运行？

template<typename T>
struct TestStruct {
    T data;
};
int main() {
    TestStruct s;
    s.data = "hello";
    return 0;
}

template<typename T>
struct Pair {
    T first, second;
};
int main() {
    Pair<char, char> p;
    p.first = 'a';
    return 0;
}

template<typename T
struct MyStruct {
    T val;
};
int main() {
    MyStruct<int> ms;
    ms.val = 3.14;
    return 0;
}

#include <string>
#include <iostream>
using namespace std;

template<typename T = string>
struct Container {
    T item;
    void setItem(T val) { item = val; }
};

int main() {
    Container<> c;
    c.setItem("Hello C++");
    cout << c.item << endl;

    Container<int> c2;
    c2.setItem(100);
    cout << c2.item << endl;
    return 0;
}