设在A，B和C单元中分别存放着三个数。若三个数都不是0，则求出三个数之和并存放于D单元中；若其中一个数为0，则把其他两个单元也清零。-In A, B and C units were placed with three numbers. If the three numbers not 0, then find the three numbers is also stored in the D unit if one number i