根据指导书给出的条件参数,若选用N个7*7的燃料组件,则平均管热力直径为多少m?
A. ((12*0.001)^2-pi*(4.2*0.001)^2)/pi/(8.4*0.001)
B. 4*((12*0.001)^2-pi*(4.2*0.001)^2)/pi/(8.4*0.001)
C. 4*((12*0.001)^2-pi*(4.2*0.001)^2)*(7*7-5)/pi/(8.4*0.001)/(7*7)
D. 4*((12*0.001)^2-pi*(4.2*0.001)^2)*(7*7)/pi/(8.4*0.001)/(7*7-5)