What is the time complexity of the following code?

public boolean isBalanced(String exp)
{
	int len = exp.length();
	Stack<Integer> stk = new Stack<Integer>();
	for(int i = 0; i < len; i++)
        {
		char ch = exp.charAt(i);
                if (ch == '(')
                stk.push(i);
                else if (ch == ')')
                {
			if(stk.peek() == null)
                        {
				return false;
			}
			stk.pop();
		}
	}
	return true;
}

A. O(logn)

B. O(n)

C. O(1)

D. O(nlogn)

Answer: Option B