以下哪段C++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;
