Two long parallel surfaces each of emissivity 0.7 are maintained at different temperatures and accordingly have radiation heat exchange between them. If is desired to reduce 75% of this radiant heat...

Two long parallel surfaces each of emissivity 0.7 are maintained at different temperatures and accordingly have radiation heat exchange between them. If is desired to reduce 75% of this radiant heat transfer by inserting thin parallel shields of same emissivity on both sides. The number of shields should be—

(a) one

(b) two

(c) three

(d) four