Yes if you have good understanding of zoology then it is, Zoology and Environmental Sciences both are scoring, I only recommended you zoology because you have already studied it and you know the zoological terms....In CE-2016 Users on this forum like Sana Rasool, Green flag and Naushan Ahmad had scored 89, 81 and 79 respectively while Nazish Hina, Sania Mushtaq, Nabeel777 and Wahab Fahad scored 59, 46, 48, 56 respectively , I dont know about greenflag much but sana rasool holds degree in Enviromental sciences likewise you also hold degree in zoology so If you know you core subject well then why not opt it and score with ease meanwhile about zoology I only found Golden Boy on this forum who scored 67 in 2016, though I dont know much about his academic background too but 67 is still a very good score.