Which of the following is a combustion reaction?
a. c3h8 (g) + 5o2 (g) → 3co2 (g) + 4h2o (g)
b. 2so3 (g) → 2so2 (g) + o2 (g)
c. 2na (s) + cl2 (g) → 2nacl (s)
d. 2kclo3 (s) → 2kcl (s) + 3o3 (g)

Find an expression for the electric field e⃗ at the center of the semicircle. hint: a small piece of arc length δs spans a small angle δθ=δs /r, where r is the radius. express your answer in terms of the variables q, l, unit vectors i^, j^, and appropriate constants.