18) C

Volume of air inhaled in 1 minute = 500 x 14= 7000cm³ = 7dm³
Volume in 4 minutes = 7 x 4 = 28dm³
Volume of oxygen inhaled in 4 min = 0.21 x 28 = 5.88 dm³
Volume of oxygen exhaled =  0.16 x 28 = 4.48 dm³
Volume of oxygen absorbed by lung capillaries = 5.88 – 4.48 = 1.4 dm³