Bit field vs Bitset

I want to store bits in an array (like structure). So I can follow either of the following two approaches

Approach number 1 (AN 1)

struct BIT
{
   int data : 1
};

int main()
{
   BIT a[100];
   return 0;
}

Approach number 2 (AN 2)

int main()
{
    std::bitset<100> BITS;
    return 0;
}

Why would someone prefer AN 2 over AN 1?