Why does the starting point of my line keep modifying?

So I am trying to make a line between me and a part where my mouse is.It is ok until i move. If I move my the line keep getting offsetted, so I don't how to fix this.

local a = Instance.new("Part",player.Character.Torso)

a.CFrame = Char.HumanoidRootPart.CFrame*CFrame.new(0,-2.5,0)

a.Anchored = true

a.Transparency = 0

a.CanCollide = false

-----------------------------------

local b = Instance.new("Part",player.Character.Torso)

b.CFrame = CFrame.new(Mouse.Hit.p.X,Mouse.Hit.p.Y,Mouse.Hit.p.Z)

b.Anchored = true

b.Transparency = 0

b.CanCollide = false

local dist = (a.Position - b.Position).magnitude

-----------------------------------

local c = Instance.new("Part",player.Character.Torso)

c.CFrame = CFrame.new(a.Position + 0.5*(b.Position - a.Position))*CFrame.new(a.Position,b.Position)

c.Anchored = true

c.Transparency = 0

c.CanCollide = false

c.Size = Vector3.new(2,1,dist)

------------------------------

game.Debris:AddItem(a, 3)

game.Debris:AddItem(b, 3)

game.Debris:AddItem(c, 3)