以下哪段代码能将数组int a[4] = {2, 4, 6, 8};的所有元素变为原来的二倍？
for (int i=0; i< 4; i++) a[i] +=2;
for (int i=0; i<4; i++) a[i] *=2;
for (int i=1; i<=4; i++) a[i] =a[i] *a[i];
for (int i=1; i<=4; i++) a[i] *=2;