$1.$ One charge $+$Q is located at the origin $(0,0),$ and a second charge $+4$Q is placed at the point $($d$,0)$ on the x$-$axis. Where should a third charge, q$,$ be placed, and what should be its sign and magnitude, so that all three charges will be in equilibrium? A$.($d$/3,0),-4$Q$/9$ B$.($d$/4,0),-4$Q$/9$ C$.($d$/3,0),4$Q$/9$ D$.($d$/3,0),-$Q$/92.$ Two point charges q$1=+2\mathrm{.}40$ nC and q$2=-6\mathrm{.}50$ nC are $0\mathrm{.}100$ m apart. Point A is in the middle point between them. Take the electric potential to be zero at infinity. Find the potential at point A$.$ A$.-737$ V B$.737$ V C$.206$ V D$.100$ V $3.$ An insulating hollow sphere has inner radius a and outer radius b$.$ Within the insulating material the volume charge density is given by $($r$)=$ $/$r$,$ where is a positive constant. In terms of and a$,$ what is the magnitude of the electric field at a distance r from the center of the shell, where a $<$ r $<$ b$?$ A$.$ $[1-($a$/$r$)2]/$$0$ B$.2$$[1-($a$/$r$)2]/$$0$ C$.$ $[1-($a$/$r$)2]/4$$0$ D$.$ $[1-($a$/$r$)2]/2$$0$