有砟道床，单线隧道小避车洞每隔（）两侧交错设置。
A.30m
B.40m
C.20m
D.50m
参考答案：A
X、Y、Z2+三种粒子具有相同的核外电子数．若三种粒子的核外电子总数为30，则Z2+的质子数为______．
The external auditing shall be conducted by the Customs over the following enterprises and other organizations
A．Enterprises and other organizations engaged in foreign trade onlyB．Enterprises and other organizations engaged in domestic trade onlyC．Enterprises engaged in the business of inward process onlyD．Both A and C
