Sandra is . . . . . . . . better dancer than . . . . . . . . singer.

The correct answer is: Sandra is a better dancer than a singer.

The reason is that the sentence is comparing Sandra's dancing ability to her singing ability. In this case, we are not comparing Sandra to a specific dancer or singer, so we use the indefinite article "a" before both "dancer" and "singer."

If we were comparing Sandra to a specific dancer or singer, we would use the definite article "the" before the noun that refers to the person we are comparing her to. For example, we could say:

• Sandra is a better dancer than the other dancers in her class.
• Sandra is a better singer than the lead singer in her band.

In these cases, we are comparing Sandra to a specific group of dancers or singers, so we use the definite article "the" before the noun that refers to the group.

I hope this explanation is helpful! Let me know if you have any other questions.