阅读理解。 These are instructions for making invisible ink using baking soda. Difficulty: Easy Time required: A few minutes What you need: Baking soda Paper Water Light bulb Paintbrush Measuring cup Purple grape juice Here's how: There are at least two methods of using baking soda as invisible ink. Mix equal amounts of water and baking soda. Use a paintbrush to write a message onto white paper, using the baking soda solution (溶液) as 'ink'. Allow the ink to dry. One way to read the message is to hold the paper up to a heat source, such as a light bulb. The baking soda will cause the writing in the paper to turn brown. The second way to read the message is to paint over the paper with purple grape juice. The message will appear in a different color. Tips: If you are using the heating method, avoid igniting (点燃) the paper. Baking soda and grape juice react with each other in an acid-base (酸碱) reaction, producing a color change in the paper. Grape juice concentrate (浓缩物) results in a more visible color change than regular grape juice. 1. ______ is not needed for making invisible ink. A. Baking soda B. Paper C. Water D. Ink 2. What's the function of the paintbrush? A. To draw a picture. B. To write a message onto the white paper. C. To allow the'ink' to appear. D. To help the'ink' dry. 3. Why will the message appear in a different color in the second way to read the massage? A. Because there is enough grape juice. B. Because baking soda reacts with grape juice. C. Because of the use of water. D. Because of the special paper. 4. From the process, we can know that making invisible ink is ______. A. very hard B. not difficult C. a little difficult D. not easy