page 12 17. A field trial was carried out to investigate the effect of applying different masses of phosphate fertiliser on grain yield of a wheat cultivar. The wheat was grown in three fields. A mass of 50, 100 or 150 kg of phosphate fertiliser per hectare was applied. How could the design of the field trial be improved to minimise the effects of bias? A Repeat the trial using a wider range of masses of phosphate fertiliser B Increase the number of replicates at each phosphate fertiliser application C Carry out another field trial exactly the same using a different wheat cultivar D Divide each field and randomly apply different masses of phosphate fertiliser 18. Fungi were cultured in a fermenter to produce an enzyme that was released into the culture medium. The graph shows the effect of oxygen concentration on the dry mass of the fungus and the concentration of enzyme in the medium after 48 hours growth. 4 3 2 1 Key fungus enzyme concentration of enzyme (units per litre) dry mass of fungus (units per litre) oxygen concentration (units) 0 30 20 10 0 60 40 20 0 The concentration of enzyme when the dry mass of fungus was 36 units per litre was A 18 units per litre B 28 units per litre C 48 units per litre D 56 units per litre.