Item a is correct. "Step by step" and "trickle" are the accumulation of things, "thousands of miles away" and "rivers and seas" are the changes of things. That is to say, quantitative change will cause qualitative change to some extent. Without quantitative change, there will be no qualitative change. Quantitative change is the necessary preparation for qualitative change, and qualitative change is the inevitable result of quantitative change. Item b is wrong. Quantitative change is a necessary preparation for qualitative change, and quantitative change is not based on qualitative change. Item c is wrong. The essence of development is the emergence of new things and the disappearance of old things, which has nothing to do with the law of mutual change of quality. Item d error. "Quality is the accumulation of quantity" is disturbing, and quantitative change is the increase or decrease of the number of things or the change of places. Qualitative change is the change of the fundamental nature of things and the interruption of the gradual process. The leap from quantitative change to qualitative change is not a simple accumulation of quantity, but a breakthrough. Therefore, choose a in this question.