The figure shows a quadrilateral ABCD, with S on AC and T on BD. For trapezoid A B C D, S and T are midpoints of the legs. If A B = x + 4, C D = 3 x + 2, and S T = 9, find A B.
Choices:
A:3
B:5
C:7
D:9
Please directly answer the question and provide the correct option letter, e.g., A, B, C, D.
Question: The figure shows a quadrilateral ABCD, with S on AC and T on BD. For trapezoid A B C D, S and T are midpoints of the legs. If A B = x + 4, C D = 3 x + 2, and S T = 9, find A B.
Choices:
A:3
B:5
C:7
D:9
Please first conduct reasoning, and then answer the question and provide the correct option letter, e.g., A, B, C, D, at the end.
Question: The figure shows a quadrilateral ABCD, with S on AC and T on BD. For trapezoid A B C D, S and T are midpoints of the legs. If A B = x + 4, C D = 3 x + 2, and S T = 9, find A B.
Choices:
A:3
B:5
C:7
D:9