杯里全是水，倒出<img style='vertical-align:middle' src="https://www.bestkids.net/android-static/%E5%85%AC%E5%8A%A1%E5%91%98/aa96ac66544f4cfebc273bb8a2cb94e9.001.png" width="15" height="41" alt="">装入纯酒精，又倒出<img style='vertical-align:middle' src="https://www.bestkids.net/android-static/%E5%85%AC%E5%8A%A1%E5%91%98/aa96ac66544f4cfebc273bb8a2cb94e9.002.png" width="16" height="41" alt="">装入纯酒精，再倒出<img style='vertical-align:middle' src="https://www.bestkids.net/android-static/%E5%85%AC%E5%8A%A1%E5%91%98/aa96ac66544f4cfebc273bb8a2cb94e9.003.png" width="15" height="41" alt="">装入纯酒精，问现在酒精浓度是多少？（　） <br>