The two numbers or letters arranged in the order are called order pairs, such as 2a, e3, dm, etc. , the first number or letter in the orordered pair is called the front item, and the second is called the post item. It is a finite sequence composed of several order pairs, where the first term of each ordered pair is a letter and a second term is a number, and for any ordered pair, if the first term is the same, the second term must be the same. According to the above definition, which of the following is a function? ['p3, c4, d6, p6, m8.', 'b3,5a, 8n, p1,66.', 'f4, h4, gm, y2, x2.', 'a3, b5, d6, p1, e3.']