下列以 C++程式語言撰寫之程式，請問①②③④中那一行的語法有誤？struct T1 { int val; }; class T2 { private: int val; }; int main() { T1 t1; //①T2 t2; //②t1.val = 10; //③t2.val = 10; //④return 0; } 
(A)①
(B)②
(C)③
(D)④