In modern society, people can hardly do without mobile phone or computer. The youth of today are even more looking forward to Apple Corp's products.    The iPad is an Apple tablet computer. The device sits somewhere between a laptop and a smart phone, according to Apple's chief executive, Steven P. Jobs, and does certain things better than both of them, like surfing the Web, reading e-books and playing videos. The iPad went on sale on April 3, 2010, and Apple said it had shipped 500,000 units in the first week. The handheld device puts Apple on a direct collision course with the Kindle from Amazon, which Mr. Jobs ever considered as pioneering the category.    It is common knowledge that media companies hope the iPad will finally lead to available ways for them to charge for news, books and other materials. The half-inch thick, 1.5-pound device features a 9.7-inch multi-touch screen and is powered by a customized Apple microchip, which is called A4. It has been designed with an exposed screen and without a camera or separate keyboard. The iPad has the same operating system as the iPhone and also has access to its 140,000 applications.    The price of the device starts at $499 for the most basic model, with a Wi-Fi wireless connection. More expensive models with more memory and with 3G wireless access will cost $629 to $829, depending on storage size. Because Apple is attempting to popularize a new kind of computing device, acceptance among consumers is expected to be slower than with previous Apple devices. Critics assume that some buyers are waiting for future versions of the iPad to appear, perhaps with a camera or USB ports .    On its first day on sale, iPad users downloaded more than one million applications from the company's AppStore and more than 250,000 electronic books from its iBookstore. A new wave of apps is expected in response to the iPad. For an application developer, having an application accepted for a highly desired Apple product means reaching a passionate group of consumers. The potential income is huge: the apps market for the iPhone and iPod Touch alone is already worth a billion dollars a year in sales. The different prices of the iPad are determined by   _  .
the size of the memory the size of the screen the speed of the microchip the Wi-Fi wireless connection