For the first question, assume that the wire is a charged straight metal rod and the linear density of charge is η. From Gauss theorem, we can calculate the electric field intensity of a single wire E=η/2πrε, and any point on the connection line is the superposition of the electric field intensities of two wires. The coordinate system is established with the center of a wire as the origin and the connecting line of two wires as the X axis (Y and Z directions are meaningless). Because the wires are equipotential bodies, it is only necessary to integrate the field strength from A to d-a when calculating the potential (I understand that D in the topic is the distance between the centers of two wires, if not, the upper and lower limits of the integration need to be changed). This is the potential difference between the two lines, defined by the capacitance, c = q/u.
For the second question, directly apply the formula W = C (U 2)/2, where U is the potential difference between two wires, V is the topic, and then substitute C..
Third, because the wire is infinitely long and the electric field has only a horizontal component, the electric field is uniform and strong in the vertical direction. Just calculate the field strength of wire A at B, and multiply it directly by η of wire, η=CV.