The usual spans with R.C.C. poles are

The correct answer is B. 60-100 meters.

R.C.C. poles are made of reinforced concrete, which is a strong and durable material. They are often used in construction, as they can support heavy loads. The usual spans with R.C.C. poles are 60-100 meters. This means that the distance between two R.C.C. poles is typically 60-100 meters.

Option A is incorrect because the usual spans with R.C.C. poles are not 40-50 meters. Option C is incorrect because the usual spans with R.C.C. poles are not 80-100 meters. Option D is incorrect because the usual spans with R.C.C. poles are not 300-500 meters.