She is probably reading too much into the questions because she's got "test anxiety." Tell her that if the question is multiple choice or true/false, she will know the answer as soon as she reads the question (if she knows the material.) Then all she has to do is look for the answer that popped into her head when she read the question. If the answer that came to her isn't among the choices given, the correct answer will be the one that requires the fewest qualifiers. In other words, if you have to use too many "if..then" to make an answer work, it's not the right answer.
If you're not dealing with multiple choice or true/false, is it a matter of her not having enough time to complete the test? Does she spend a lot of time on some questions that may be more difficult and then not get to the rest of the test? If that's the case, tell her to look at the question and decide if she knows the answer or can figure it rather quickly. With those, she should answer them and move on. With those that she immediately knows are going to take more time, she should make a little mark and move on to the next one. When she's done with all that were easier, she can come back to the ones that she felt would take more time.
Also to be considered, is there any possibility that there is a reading problem -- that the teacher is talking through the morning work and so your daughter is able to follow along but, when the test comes, she has to read and interpret the entire thing on her own? Could some form of dyslexia be at work here that no one has noticed because she is doing relatively well? Remember, countless people with learning disorders have made it through school because they have developed methods that enabled them to maneuver through the system undetected.
Lastly, what's wrong with a B? Didn't Einstein actually flunk math? Good luck, A..