Math prep No 129 求助

If t is a positive integer and r is the remainder when t² +5t+6 is divided by 7,what is the value of r?

1)when t is divided by 7,the remainder is 6

2) when t² is divided by 7,the remainder is 1

the key is A

为什么不是D 啊? 由二式中t² is divided by 7,the remainder is 1 可得t 是6 ，r的值是2