He was not afraid to pet the gentle dog even though it was very big.

Giải thích: gentle (a): nhẹ nhàng, dịu dàng

dirty (a): bẩn                                                                          cold (a): lạnh

calm (a): bình tĩnh                                                                 fierce (a): hung dữ

=> gentle >< fierce

Tạm dịch: Anh ấy không sợ nuôi con chó hiền lành mặc dù nó rất to.