Read the following article and answer the question. Article: Does it drive you crazy that your cell phone needs to be frequently charged for hours? A team of scientists led by professor Harold Kung at Northwestern University in the US may have solved your problem. They developed a lithium ion batery that holds 10 times as much power and charges 10 times more quickly than standard batteries, according to the BBC in a report on November 15. Lithium ion bateries are rechargeable and are widely used in cameras, smartphones and laptops. They charge through a chemical reaction in which lithium ions are sent between the two ends of a battery. How much power the battery holds and how fast it charges are limited by two things:how many lithium ions it has and how quickly these ions move. Current rechargeable batteries have an anode made of many carbon-based graphene sheets. To increase energy capacity, former experiments have tried to replace the carbon with silicon, which can hold far more lithium ions. However, this method did not work because the silicon was not stable enough. Kung's team managed to stabilize the silicon. They put clusters of silicon between the graphene sheets like a sandwich so that the silicon could not move around freely and take away the energy. The speed at which a battery charges is hindered by the shape of the graphene sheets because it takes a long time for the lithium to travel from one side of the sheet to the other.Sometimes a "traffic jam" occurs around the edges of the graphene. To solve this problem, Kung's team used a special chemical process to create tiny holes of 10 to 20 nanometers wide in the graphene sheets so the lithium ions would have a "shortcut" and be able to travel directly to the other side. This reduced the time it took the battery to recharge. About 15 minutes of charging can last more than a week. "Even after 150 charges, which would be one year or more of operation, the battery is still five times more effective than lithium ion batteries on the market today," said Kung. The technology could be widely available... Question: To solve "traffic jam" while the battery charges, Kung's team _ . Answer:

(a). fixed silicon between the graphene sheets. (b). created tiny holes in the graphene sheets. (c). made the graphene sheets in special shapes. (d). used a special chemical process to make more efficient graphene sheets.
Answer:
(b).