Ozonised oxygen is prepared in the laboratory by passing a silent electric discharge through cold, dry dioxygen. A silent discharge rather than a spark is used because

1. Aa spark would convert $\mathrm{O}{}_2$ into $\mathrm{O}$ atoms only, which cannot form ozone
2. Ba silent discharge gives pure ozone
3. Cmoisture is formed during a spark and reacts with the catalyst
4. Da spark produces local heating which decomposes ozone back into dioxygen; the usual product contains about 10% ozone